P2: PHENIX build probe leaks parent symlink-loop exceptions #960

Severity: P2. Fresh-context review of PR #957 at f37a017dcd6d5b4c8cd769d7e0280345a65d2789; independently reproduced on the required Python 3.12 by PRIMARY.

scripts/toolchain.py:337 resolves the dispatcher target's parent path. A directory symlink loop there raises RuntimeError on Python 3.12. The outer phenix_build_evidence() exception handler at line 661 catches only OSError, ValueError and subprocess.TimeoutExpired, so the exception escapes instead of returning diagnostic unknown/non-reusable evidence.

Temporary reproduction: prefix/loop -> loop and prefix/phenix_bin/phenix.version -> ../loop/phenix.version. Calling phenix_build_evidence() raises RuntimeError: Symlink loop from .../prefix/loop. No setup or real vendor command was executed. This is distinct from #958's outside-hop admission: here an invalid path aborts callers rather than producing the promised unknown receipt.

Suggested fix: classify resolution loops at the path boundary and retain a diagnostic error in the build evidence, without swallowing unrelated implementation errors. Add hermetic loop regressions for affected configured paths and both vendor/bare probes, proving no setup/dispatcher runs and the measured-build gate remains false.

Triage: in scope, because invalid configured installation paths must fail closed with retained evidence under the new adapter contract. Deduplicated against the current queue and all-state symlink-loop search; no matching issue. Local receipt: .orchestration/evidence/029-primary-reproductions.json.

The fresh reviewer also verified this RuntimeError escaped on the base implementation. It is a pre-existing limitation, not a new regression; fixing it is in scope while revising this path boundary and its unknown-evidence contract.