fix: pick the glibc SDK on glibc hosts, and stop skipping the suite (#215)

[s2x]

Follow-up to the #215 work on main. Two problems there made local development on a glibc machine impossible. Both were hit while trying to run the milestone-0.7 issues.

1. fetch_zvec_sdk.sh picked the musl SDK

is_musl() { ls /lib/ld-musl-*.so.1 >/dev/null 2>&1; }

On any glibc host with musl-tools installed that succeeds. The musl SDK got unpacked, and every test failed at ZVec::ffi():

FFI\Exception: Failed loading '.../libzvec_ffi.so'
  (libc.musl-x86_64.so.1: cannot open shared object file: No such file or directory)

$ objdump -p sdk/lib/libzvec.so | grep NEEDED
  NEEDED   libc.musl-x86_64.so.1     # while PHP here is a plain glibc build
$ ldd $(command -v php) | grep -c musl
0

The property that matters is not what the machine has installed but which loader the PHP binary uses, so detection now inspects ldd $(command -v php), falls back to /bin/sh when there is no PHP, and assumes glibc when neither marker is present. PHP_BINARY overrides the probe.

The SDK layout then hid it. copy_if_different in ffi/CMakeLists.txt keeps the previously copied libzvec.so when the incoming one has a different mtime, so a stale musl libzvec.so sat next to a freshly linked adapter and the adapter built and linked fine. rm -rf sdk ffi/build is needed when switching SDKs — now documented.

2. The documented test command skipped 172 of 191 tests

AGENTS.md mandated php run-tests.php -n tests/. -n strips php.ini, and FFI comes from conf.d/20-ffi.ini here rather than being compiled in:

$ php -n -r 'var_dump(extension_loaded("ffi"));'
bool(false)

So the whole suite reported SKIP ... reason: FFI extension not available, and the 16 tests that did run passed — a green-looking summary over almost nothing:

	before	after
passed	16	189
skipped	172	0
failed	1	0

The real goal is FFI enabled and the legacy zvec extension disabled, which is php run-tests.php -n -d extension=ffi.so tests/. Updated everywhere, plus a note that a run reporting skips is a broken run rather than a pass.

Verification

From a clean tree:

rm -rf sdk ffi/build && ./build_zvec.sh
php run-tests.php -n -d extension=ffi.so tests/

Number of tests : 191               191
Tests skipped   :   0 (  0.0%)
Tests failed    :   0 (  0.0%)
Expected fail   :   2 (  1.0%)
Tests passed    : 189 ( 99.0%)

test_dbs/ is left with only .gitignore.

Note

The Mismatched free() / delete finding from #228 is still open and untouched here.

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