…ependent (#24565)

The `Allocations` CI job fails whenever the snapshots are regenerated on a different CPU architecture than CI (e.g. https://github.com/oxc-project/oxc/actions/runs/29420345285/job/87368903834): every `arena size` value differs by 32–128 bytes between aarch64 and x86_64. The cause is target-dependent type layout — hashbrown's table control groups are 16 bytes wide on x86_64 (SSE2) but 8 bytes on aarch64 (NEON) — so each live `HashMap` in the arena shifts `used_bytes()` slightly. This is the same root cause that #22621 handled for chunk counts; the `arena size` gauge added in #24553 re-exposed it. Allocation counts are unaffected and stay exact.

Exact byte equality across architectures isn't achievable (the arena genuinely occupies a different number of bytes), so instead the snapshot writer tolerates layout noise:

- Snapshots are now YAML (`allocs_*.yaml`) with the same layout as before: one value per line, file names at column 0 for git hunk headers, exact numbers as the diffed values, and human-readable sizes moved to trailing `#` comments.
- When regenerating, `cargo allocs` parses the committed snapshot (with `saphyr`, already a workspace dependency used by `oxc_coverage`) and keeps the committed `arena size` when the measured value is within `ARENA_SIZE_TOLERANCE` (1024 bytes, ~8× the observed worst-case cross-arch drift). Differences beyond the tolerance — real regressions such as growing an AST node type change sizes by orders of magnitude more — rewrite the value and fail the `git diff --exit-code` check as before.
- The trailing comments are rendered from the recorded value, so a snapshot regenerated on any platform stays byte-identical while within tolerance. No CI workflow changes are needed, and drift can't accumulate silently: each run compares against the committed value, so creeping sub-tolerance changes eventually cross the threshold and surface.

The committed values here are aarch64-measured; x86_64 CI measures 32–128 bytes higher and keeps them. Open PRs that touch the old `.snap` files will need a rebase and a `cargo allocs` rerun.

🤖 Implemented with [Claude Code](https://claude.com/claude-code).