runtime: fast-json-stringify triggers stale ShapeId facts

[proggeramlug]

Release blocker

Full CI run 33264050779 fails deterministically in cargo-test-perry shard 1 on candidate 2705439.

Reproduction

PERRY_REQUIRE_NPM_E2E=1 cargo test -p perry --test issue_6559_real_libs_e2e real_fast_json_stringify_serializer -- --exact --nocapture

The dedicated Linux server reproduces it in about 161 seconds; macOS reproduces it in about 253 seconds. The compiled fast-json-stringify@7 fixture aborts at crates/perry-runtime/src/object/shapes.rs with:

published ShapeId disagrees with authoritative ObjectHeader facts

Disproved hypothesis

Disabling the in-place delete path from #9013 and restoring unconditional clone-on-delete does not fix the failure. The same exact assertion still fires, so #9013 is not the root cause.

Required diagnosis and acceptance

• Report the stale ShapeId plus descriptor keys/count and authoritative keys/count at the assertion.
• Identify the mutation/publication path that leaves the immutable descriptor stale.
• Add a network-independent focused regression when the path is known.
• The exact fast-json-stringify test must pass on the dedicated server.
• Runtime shape tests, exact-SHA Full CI, and exact-SHA Simulator must pass before release.

[proggeramlug]

Fixed on release/v0.5.1519 by 40d2c5f.

Root cause and validation:

• try_birth_stamp_preinstalled_shape trusted pointer identity without validating the canonical keys array's current physical count; the failing object had a one-key descriptor over the same array after its length became zero. The fast path now fails closed and uses exact publication.
• The real-libraries harness ignored CI's release runtime and rebuilt a debug/panic=unwind archive, contrary to Perry's production exception transport. It now honors PERRY_RUNTIME_DIR and otherwise builds --release.
• Dedicated Linux server: all three focused preinstalled-shape tests passed.
• Production-equivalent exact test passed: PERRY_RUNTIME_DIR=.../target/release PERRY_REQUIRE_NPM_E2E=1 cargo test -p perry --test issue_6559_real_libs_e2e real_fast_json_stringify_serializer -- --exact --nocapture (1 passed, 0 failed).