[swjnxyf]

Summary

Fix the Build Identity fixture tests for Cargo 1.100's changed intermediate build-directory layout, without changing production code.

• Read the canonical OUT_DIR from Cargo's build-script-executed JSON message instead of hard-coding target/debug/build/identity-fixture-*/output.
• Resolve the captured build-script stdout using the legacy output or new run/stdout form relative to that Cargo-provided directory.
• Retain all assertions for packed refs without reflogs, linked worktrees, same-HEAD dirty tracking, exact build-script output timestamps, and no-op rebuilds.

Why

Related to #1420: its CI run #43710045930 has two independent originating failures (test-linux-tooling and contract) in unchanged scripts/tests/test_build_identity.py, which then cause four aggregate gate failures.

The failing CI uses Ubuntu 26.04 and Cargo 1.100.0; the previous successful main CI used Ubuntu 24.04 and Cargo 1.99.1. Cargo 1.100 relocates build-script output to <build-dir>/debug/build/<pkg>/<unit>/run/stdout, while the test assumed the pre-1.100 <build-dir>/debug/build/<pkg>-<hash>/output layout.

Verification (special)

• Before the fix, RUSTUP_TOOLCHAIN=1.100.0 reproduced the same four CI assertion failures.
• After the fix, python3 -m unittest scripts.tests.test_build_identity -v passes 5/5 with both Rust/Cargo 1.95.0 and 1.100.0.
• RUSTUP_TOOLCHAIN=1.100.0 PYTHONDONTWRITEBYTECODE=1 bash scripts/release_check.sh --static-only passes, including 904 Python tests (32 skipped), npm installer/package smoke, and release static contracts.
• git diff --check passes.

This PR is separate from #1420's Wake changes because #1420 originates from a fork with maintainer_can_modify=false; the maintainer cannot push a follow-up commit directly to that branch. Once merged into main, the updated base can be used to revalidate #1420. Keep its required CI checks enforced rather than bypassing them.