fix(rust): the toolchain's children are told where the toolchain is

[CesarManzoCode]

The VM closed the previous cause: inside Thalyx, cargo 1.90.0 from: thalyx and rust-analyzer 1.90.0 from: thalyx. And the machine still could not resolve a name. context('lantern/src/lib.rs') found LanternRegistry; context('LanternRegistry') answered resolution: nothing, entries: [], and a rename at the identifier's own physical position — 8:12, not the outline's guess — answered No references found at position.

Reproduced here with an environment holding exactly what Thalyx hands over and nothing else, and the cause is in the server's own log:

ERROR FetchWorkspaceError: rust-analyzer failed to load workspace:
  Failed to run `cargo metadata …`: No such file or directory (os error 2)
WARN  failed to get rustc cfgs e=unable to fetch cfgs via `… "rustc" …`
  Caused by: No such file or directory (os error 2)

rust-analyzer is not a program Thalyx runs; it is a program that runs programs. It spells cargo metadata, cargo locate-project, cargo --version and rustc --print cfg as bare names, which the kernel resolves through PATH and nothing else — and Thalyx, which finds every tool by absolute path and goes on doing so, had never given its children one. With no crate graph there is no semantics; syntax survives because syntax needs no subprocess, which is exactly the shape the machine showed.

The change is one variable. toolchain::environment now hands a managed toolchain a PATH built by Thalyx out of one directory, its own bin: nothing inherited, no /usr/bin, no ~/.cargo, no ~/.rustup. Measured — adding it and nothing else turned workspace/symbol, definition and rename from empty to correct in the same fixture. CARGO, RUSTC and RUST_SRC_PATH are deliberately not set: PATH alone was sufficient, and a variable added because Rust usually has it is a variable nobody can remove later. An installed toolchain gets its bin in front of the inherited PATH rather than instead of it, which is also what sudo's secure_path had been costing verify.sh.

Two instrument defects came with it, and both were worse than the bug.

dev/verify-agent-rust.sh printed PROVEN context('LanternRegistry') came from rust-analyzer about {source: "rust-analyzer", resolution: "nothing", entries: []}. source says who answered; it has never said that the answer resolved anything. PROVEN now needs all three: rust-analyzer answered, resolution is one, and the entries carry exactly the declaration asked about, with the handle a rename would take.

And the control written to demonstrate the defect resolved the symbol anyway. rust-analyzer looks for cargo in three places — $CARGO, PATH, $CARGO_HOME/bin — and a rustup machine has the third one full. Inside Thalyx CARGO_HOME is /state/cargo and has no bin, so the control was modelling a developer's laptop rather than the guest. It models the guest now.

Separately, the outline placed a documented struct at its doc comment: with hierarchicalDocumentSymbolSupport: false each entry carries one range and rust-analyzer fills it with the whole item, so LanternRegistry came back at 3:1 instead of 8:12. The hierarchical answer carries selectionRange, which is the identifier. Not the blocker — 8:12 failed too — but a position a rename by handle would have used. The server's real answer is captured verbatim in tests/samples/document-symbol-hierarchical.json, rule 6.

Proven in the container, with env_clear and only Thalyx's pairs, over dev/rust-corpus: without PATH the outline finds the struct and every name resolves to nothing; with it, Resolution::One at lantern/src/lib.rs:8:12, uses in the other crate, and a real rename across two files. And the chain below — cargo invoked as a bare name with an empty environment compiles the workspace, so it found its rustc the same way.

Confinement is untouched on purpose. The VM reported analyzer_confined=false because the kernel is attached but observing, and the provider fell back to an ordinary process inside Thalyx and the semantics failed anyway. One cause at a time.

Claude-Session: https://claude.ai/code/session_01S4ij9nSjMTiCT6sd8DuBgQ