No-GIL debugger: make script and statement registries race-free

[chrisbbreuer]

CI: https://github.com/zig-utils/zig-js/actions/runs/29674163787/job/88158274698
Roadmap: #143
Related: #293

Root cause

The no-GIL PR-249 case jit/int-gate-stop-budget.js produced a
suppression-free TSan race between:

• registerParsedDebugScript mutating the shared
  Context.debug_statement_locations AutoHashMap while a child constructs a
  dynamic Function; and
• serviceDebugStatement reading the same map from another child.

The same transaction also mutates debug_scripts and
next_debug_script_id unsafely, so fixing only the reported map access would
leave duplicate IDs/list corruption latent.

Acceptance

• Serialize script ID assignment, script-list publication, notify-hook
  installation/removal, location-map mutation, and all concurrent reads.
• Keep single-threaded and GIL execution on the lock-free path.
• Never expose a partially registered script/location record.
• Add a deterministic multi-thread dynamic-Function witness with exact
  script-ID uniqueness and result checks.
• Pass focused normal and suppression-free TSan tests.
• Re-run the exact jit/int-gate-stop-budget.js TSan witness.
• Observe a green replacement full CI shard before closing.

Local evidence

• zig build test -Dtest-filter="debug registry": 3/3 passed.
• zig build test -Dtest-filter="debug registry" -Dtsan=true: 3/3 passed,
  suppression-free.
• zig build threads-test -Dthreads-parallel-js=true -Dthreads-case=jit/int-gate-stop-budget.js -Dtsan=true: 1/1 passed in
  26.3 seconds.
• zig build test -Dtest-filter=inspector: 20/20 passed.

[chrisbbreuer]

Landed on main as c0ea1fa.

Validation:

• zig build test -Dtest-filter="debug registry": 3/3 passed
• zig build test -Dtest-filter="debug registry" -Dtsan=true: 3/3 passed, suppression-free
• exact no-GIL TSan corpus witness: jit/int-gate-stop-budget.js passed 1/1 in 26.3s
• zig build test -Dtest-filter=inspector: 20/20 passed

The replacement full-CI result remains the only open acceptance item.

[chrisbbreuer]

Replacement run 29697559754 passed all four normal unit shards and the suppression-free tsan-parallel-js job on 7650986f. The shared script/location registry race is no longer present.