Revert "chore(xlings): pin 2026.8.17.2 -> 2026.8.27.2"

The bump is correct and this release cannot carry it. Measured on CI rather than
predicted:

    Downloading xim:glibc@2.44
    error: toolchain post-install fixup: selected RuntimeBinding glibc@2.44.2
           requires payload '…/xpkgs/xim-x-glibc/2.44.2', but it is not installed

Two versions of one decision meet inside a single run. The mcpp that BUILDS this
one is the released 2026.8.25.2, whose compiled-in pin is 2026.8.17.2, and it
installs the payload its own binding names: 2.44. The vendored xlings that
COMPUTES the binding is the one this branch pins, which asks the index and gets
2.44.2. The request is then newer than what is installed.

That is the direction `payload_dir_for_version` refuses, and it must: accepting a
payload older than the request would silently use the one the index states is not
the one to install --- the preload defect of mcpp#484.

⚠️ THIS IS NOT A STALE CACHE. The sandbox cache key includes the xlings version
for exactly this reason, so the bump is a cache miss and the sandbox was fresh.
The mismatch is between the engine doing the building and the xlings it was told
to vendor.

⭐ IT IS SELF-HEALING, AND ONLY IN ONE ORDER. Once 2026.8.27.1 is released, a
bootstrap from it carries the new pin, computes 2.44.2, and installs 2.44.2. The
bump therefore belongs in the release AFTER this one, which is the rule the index
states in its own source: "The index is DATA and the client is a PROGRAM: the
consumer ships first."

The engine-side half of the same problem stays in this release:
`payload_dir_for_version` accepts an installed payload whose version REFINES a
request an older client already compiled in, so older clients stop being stranded
by an index move in the safe direction.

Author: speak-agent

.github/actions/bootstrap-mcpp/action.yml

@@ -25,7 +25,7 @@ inputs:
     # `package.name`, so one of the two was simply unreachable — and which one
     # depended on the machine, which is why CI failed on `compat:lua` on
     # Windows and `mcpplibs.capi:lua` on Linux. Never pin below that.
-    default: '2026.8.27.2'
+    default: '2026.8.17.2'
   cache-target:
     description: also restore/save target/ (build artifacts + BMIs)
     required: false

.github/actions/setup-macos-llvm/action.yml

@@ -15,7 +15,7 @@ inputs:
     # Floor imposed by the index, not a routine bump — see
     # .github/actions/bootstrap-mcpp/action.yml for why 0.4.69 is required
     # (two packages named `lua` in one repo need openxlings/xlings#381).
-    default: '2026.8.27.2'
+    default: '2026.8.17.2'
 
 runs:
   using: composite

.github/workflows/bootstrap-macos.yml

@@ -17,7 +17,7 @@ jobs:
       # Dormant (workflow_dispatch only), but kept in step with the rest —
       # check_version_pins.sh holds it there. Floor: 0.4.69, below which the
       # index cannot resolve two packages that share a short name.
-      XLINGS_VERSION: '2026.8.27.2'
+      XLINGS_VERSION: '2026.8.17.2'
     steps:
       - uses: actions/checkout@v4
 

.github/workflows/ci-fresh-install.yml

@@ -152,7 +152,7 @@ jobs:
         env:
           XLINGS_NON_INTERACTIVE: '1'
         run: |
-          curl -fsSL https://raw.githubusercontent.com/openxlings/xlings/main/tools/other/quick_install.sh | bash -s v2026.8.27.2
+          curl -fsSL https://raw.githubusercontent.com/openxlings/xlings/main/tools/other/quick_install.sh | bash -s v2026.8.17.2
           echo "$HOME/.xlings/subos/current/bin" >> "$GITHUB_PATH"
 
       - name: Install mcpp and config mirror
@@ -293,7 +293,7 @@ jobs:
 
       - name: Install xlings + mcpp
         run: |
-          curl -fsSL https://raw.githubusercontent.com/openxlings/xlings/main/tools/other/quick_install.sh | bash -s v2026.8.27.2
+          curl -fsSL https://raw.githubusercontent.com/openxlings/xlings/main/tools/other/quick_install.sh | bash -s v2026.8.17.2
           # Deliberately NOT writing to $GITHUB_PATH here. On container
           # images that declare no PATH in their config (opensuse/
           # tumbleweed), appending a single dir to GITHUB_PATH makes the
@@ -364,7 +364,7 @@ jobs:
           #             (older ones carry minos=15 and refuse to start).
           #   v0.4.51+: in-process sha256 — this image has no sha256sum
           #             binary, so pinned fetches failed before it.
-          curl -fsSL https://raw.githubusercontent.com/openxlings/xlings/main/tools/other/quick_install.sh | bash -s v2026.8.27.2
+          curl -fsSL https://raw.githubusercontent.com/openxlings/xlings/main/tools/other/quick_install.sh | bash -s v2026.8.17.2
           echo "$HOME/.xlings/subos/current/bin" >> "$GITHUB_PATH"
 
       - name: Install mcpp and config mirror

.github/workflows/ci-linux-e2e.yml

@@ -237,7 +237,7 @@ jobs:
 
       - name: Bootstrap xlings + released mcpp
         run: |
-          curl -fsSL https://raw.githubusercontent.com/openxlings/xlings/main/tools/other/quick_install.sh | bash -s v2026.8.27.2
+          curl -fsSL https://raw.githubusercontent.com/openxlings/xlings/main/tools/other/quick_install.sh | bash -s v2026.8.17.2
           export PATH="$HOME/.xlings/subos/current/bin:$PATH"
           xlings update
           xlings install mcpp -y -g

.github/workflows/cross-build-test.yml

@@ -122,7 +122,7 @@ jobs:
           # release assets were uploaded in a broken state (records present,
           # blobs missing → 404 on GET); re-uploaded clean. The stale-INDEX
           # half is handled by the marker-clear below.
-          XLINGS_VERSION: '2026.8.27.2'
+          XLINGS_VERSION: '2026.8.17.2'
         run: |
           tarball="xlings-${XLINGS_VERSION}-linux-x86_64.tar.gz"
           bash "$GITHUB_WORKSPACE/.github/tools/fetch_release.sh" \
@@ -263,7 +263,7 @@ jobs:
       - name: Bootstrap mcpp via xlings
         env:
           XLINGS_NON_INTERACTIVE: '1'
-          XLINGS_VERSION: '2026.8.27.2'
+          XLINGS_VERSION: '2026.8.17.2'
         run: |
           tarball="xlings-${XLINGS_VERSION}-linux-x86_64.tar.gz"
           bash "$GITHUB_WORKSPACE/.github/tools/fetch_release.sh" \

.github/workflows/release.yml

@@ -96,7 +96,7 @@ jobs:
           # Pin xlings to a known-good version. The upstream install
           # script always grabs `latest` (no version override), so we
           # download + self-install manually to avoid broken releases.
-          XLINGS_VERSION: '2026.8.27.2'
+          XLINGS_VERSION: '2026.8.17.2'
         run: |
           if [ ! -x "$HOME/.xlings/subos/default/bin/xlings" ]; then
             tarball="xlings-${XLINGS_VERSION}-linux-x86_64.tar.gz"
@@ -289,7 +289,7 @@ jobs:
       - name: Bootstrap mcpp via xlings
         env:
           XLINGS_NON_INTERACTIVE: '1'
-          XLINGS_VERSION: '2026.8.27.2'
+          XLINGS_VERSION: '2026.8.17.2'
         run: |
           tarball="xlings-${XLINGS_VERSION}-linux-x86_64.tar.gz"
           bash "$GITHUB_WORKSPACE/.github/tools/fetch_release.sh" \
@@ -360,7 +360,7 @@ jobs:
           # below are pinned to the same version as XLINGS_VERSION; they are
           # NOT interpolated from it, so check_version_pins.sh scans for them
           # explicitly (they were absent from the old lock-step comment).
-          XLA="xlings-2026.8.27.2-linux-aarch64.tar.gz"
+          XLA="xlings-2026.8.17.2-linux-aarch64.tar.gz"
           # NOT fetch_release.sh: this asset is OPTIONAL and the `if` is the
           # point — an arch with no prebuilt xlings must fall through quietly,
           # while the helper retries a 404 five times before giving up. The one
@@ -369,9 +369,9 @@ jobs:
           # cover it.
           if curl -fsSL --retry 3 --retry-delay 2 --retry-all-errors \
                --connect-timeout 20 --max-time 600 -o "/tmp/$XLA" \
-               "https://github.com/openxlings/xlings/releases/download/v2026.8.27.2/$XLA"; then
+               "https://github.com/openxlings/xlings/releases/download/v2026.8.17.2/$XLA"; then
             tar -xzf "/tmp/$XLA" -C /tmp
-            XLBIN=$(find /tmp/xlings-2026.8.27.2-linux-aarch64 -path '*/bin/xlings' -type f | head -1)
+            XLBIN=$(find /tmp/xlings-2026.8.17.2-linux-aarch64 -path '*/bin/xlings' -type f | head -1)
             if [ -n "$XLBIN" ]; then
               mkdir -p "$STAGING/$WRAPPER/registry/bin"
               cp "$XLBIN" "$STAGING/$WRAPPER/registry/bin/xlings"
@@ -449,7 +449,7 @@ jobs:
       - name: Bootstrap mcpp via xlings
         env:
           XLINGS_NON_INTERACTIVE: '1'
-          XLINGS_VERSION: '2026.8.27.2'
+          XLINGS_VERSION: '2026.8.17.2'
         run: |
           if [ ! -x "$HOME/.xlings/subos/default/bin/xlings" ]; then
             WORK=$(mktemp -d)
@@ -632,7 +632,7 @@ jobs:
         shell: bash
         env:
           XLINGS_NON_INTERACTIVE: '1'
-          XLINGS_VERSION: '2026.8.27.2'
+          XLINGS_VERSION: '2026.8.17.2'
         run: |
           # Captured before the `cd` below, in POSIX form: this step never
           # returns to the workspace, and GITHUB_WORKSPACE is a backslash

src/platform/xlings/xlings.cppm

@@ -45,7 +45,7 @@ namespace pinned {
     // in lock-step by hand; that list was already missing both composite
     // actions, which is how CI's sandbox sat on 0.4.30 unnoticed while
     // everything else had moved on. Don't reintroduce a hand-maintained list.
-    inline constexpr std::string_view kXlingsVersion   = "2026.8.27.2";
+    inline constexpr std::string_view kXlingsVersion   = "2026.8.17.2";
     inline constexpr std::string_view kNasmVersion     = "3.02";
 }