otel-thread-ctx: add ThreadContext.invalidate() to flip valid byte

[szegedi]

Summary

Adds ThreadContext.invalidate() to the OTEP-4947 writer's addon-exposed API. Writes 0 into the record's valid header byte in place, using the same volatile + atomic_signal_fence protocol the constructor and Append() already use for header bytes an out-of-process reader can race with.

Why

When an SDK finishes a span, clearContext() on the current async-context frame detaches the ThreadContext only from that frame. Sibling and detached-continuation frames that already inherited the reference keep holding the same JS object — and with it the same underlying native record — so an eBPF reader sampling those contexts still sees the finished span's trace / span IDs as active. This is admittedly rare, but a corner case worth addressing.

Every async-context frame holding the same ThreadContext reference observes the same shared record buffer, so a single invalidate() drops the record out of scope for every such frame at once. Readers see valid=0 and MUST ignore the record per OTEP-4947.

Addresses Codex review point #2 on DataDog/dd-trace-js#9210 ("Clear inherited contexts after finish"). The dd-trace-js follow-up wiring onSpanFinished → cached.context.invalidate() will land in a separate PR after this addon change ships.

Same change ported to upstream at polarsignals/custom-labels (branch `otel-thread-ctx-invalidate`).

Test plan

• Local `npm run compile` + `npm run rebuild` clean.
• 3 new mocha cases: valid byte flips in place, idempotent, appendAttributes after invalidate mutates attrs_data but keeps valid=0.
• Docker test suite currently fails on `origin/main` too (`Cannot find module './heap-profiler-bindings'`) — unrelated to this change; will unblock via a separate fix.

Jira: PROF-15665

[github-actions]

Overall package size

Self size: 2.46 MB
Deduped: 3.16 MB
No deduping: 3.16 MB

Dependency sizes

| name | version | self size | total size | |------|---------|-----------|------------| | pprof-format | 2.3.0 | 503.97 kB | 503.97 kB | | source-map | 0.8.0 | 185.66 kB | 185.66 kB | | node-gyp-build | 4.8.4 | 13.86 kB | 13.86 kB |

_{🤖 This report was automatically generated by heaviest-objects-in-the-universe}