CI: don't hard-fail on the confirmed TSan-can't-start-here runner limitation

The real Gitea Actions run on this project's own registered runner just
failed exactly as CONTRIBUTING.md already anticipated it might:
FATAL: ThreadSanitizer: unexpected memory mapping, the same signature
already documented as a sandbox/container seccomp restriction blocking
personality(ADDR_NO_RANDOMIZE), which TSan needs to start at all. Build,
test suite, and ASan/UBSan all passed -- only the TSan step failed, on an
environment issue, not a code issue.

Fixed the CI step itself rather than just noting the failure: it now
classifies each binary's TSan run as PASS, a known flake (that exact
FATAL signature and nothing indicating an actual race was found), or a
real failure (anything else -- a genuine data race, a crash, any other
error). Only a real failure fails the build. Verified the classification
logic locally against four cases (a synthetic real race report, a plain
assertion failure, the known flake signature alone, and a clean pass) --
each classified correctly -- and against this sandbox's own six test
binaries, all six of which hit the real flake (this sandbox has never
been able to run TSan either) and correctly did not fail the build.

This does NOT mean TSan verification is happening in CI -- it means CI no
longer conflates "TSan couldn't start" with "the build is broken."
Updated CONTRIBUTING.md and CLAUDE.md from "whether the runner can execute
TSan is unverified" (a hedge) to the now-confirmed fact that it can't, and
corrected an overclaim in README.md that the suite is "regularly run under
ThreadSanitizer" -- as far as this project has been able to confirm, TSan
has not actually completed a run in any environment it's been built in
yet, local sandbox or CI. ASan/UBSan remain the real, run, load-bearing
sanitizer coverage.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UqJpkdJ6Njnt1pw3CbghzB

CLAUDE.md

@@ -19,7 +19,7 @@ A single test: `make build/test_mem && ./build/test_mem` (substitute any `test_*
 
 Sanitizer builds are not wired into `make test` (they need per-file compilation with sanitizer flags plus `-D_GNU_SOURCE -Iinclude -Isrc`); see `.gitea/workflows/ci.yml` (Gitea Actions) for the exact invocation, which CI runs on every push. **Any change to `upper.c`, `overlay.c`, or `vfs.c` must be verified under `-fsanitize=address,undefined` and `-fsanitize=thread` before being considered done** — this is not a formality: exactly this process caught a real use-after-free in the snapshot-reclamation logic during initial development (see the `reclaim_gate` note below), which the plain build and even repeated plain test runs never surfaced.
 
-**If your environment cannot run ThreadSanitizer at all, say so rather than skipping it silently.** TSan needs `personality(ADDR_NO_RANDOMIZE)` to disable ASLR for itself; some sandboxes block that syscall outright, in which case *every* TSan build fails identically (`FATAL: ThreadSanitizer: unexpected memory mapping`), including a trivial unrelated pthread program — that is the confirming test, not a PackFS-specific symptom. In that situation, ASan/UBSan are still run and still matter (they are what actually caught the `reclaim_gate` bug above), but they do not perform TSan's happens-before race analysis and are not a substitute for it; a change is TSan-verified only once it has passed on a machine or CI run that can actually execute it, not merely because ASan/UBSan passed.
+**If your environment cannot run ThreadSanitizer at all, say so rather than skipping it silently.** TSan needs `personality(ADDR_NO_RANDOMIZE)` to disable ASLR for itself; some sandboxes block that syscall outright, in which case *every* TSan build fails identically (`FATAL: ThreadSanitizer: unexpected memory mapping`), including a trivial unrelated pthread program — that is the confirming test, not a PackFS-specific symptom. **This project's own CI runner is one of them, confirmed by its first real run** (see `CONTRIBUTING.md` for the details and for exactly how `.gitea/workflows/ci.yml`'s TSan step now tells that specific, known failure apart from a real finding rather than either hiding it or failing the build over it) — a green CI run is therefore not proof TSan executed. In that situation, ASan/UBSan are still run and still matter (they are what actually caught the `reclaim_gate` bug above), but they do not perform TSan's happens-before race analysis and are not a substitute for it; a change is TSan-verified only once it has actually passed on a machine confirmed able to execute it, never merely because ASan/UBSan passed or because a CI step reported success.
 
 **A related but separate flake affects ASan/UBSan themselves in that kind of sandbox, not just TSan:** a sanitizer-built test binary can non-deterministically fail to start with `AddressSanitizer:DEADLYSIGNAL`, occasionally as an unbounded repeating loop rather than a single line — a sandbox startup race, confirmed by it hitting different, unrelated binaries across repeated runs, each of which then passes cleanly on retry. See `CONTRIBUTING.md`'s workflow section for the full description and the required mitigation (`timeout`-wrap sanitizer runs in such an environment; treat `DEADLYSIGNAL` alone, without an actual `ERROR: AddressSanitizer` or `runtime error:` string, as inconclusive and re-run rather than as a finding).