fix(msan): reports print with fast unwinding; five suites run again

Five suites (grammar_regression grammar_labels pipeline lang_contract
grammar_probe_e) were excluded from the MSan lane for "stack-overflow
under instrumentation". That was never deep recursion. MSan aborted while
REPORTING real uninitialized reads:

- every abort was on the main thread. MSan's "T<n>" is the OS thread id
  and equalled the pid in all five x86-64 aborts (CI job 91868259558);
  the old block read it as a worker-thread ordinal, so the worker-stack
  knob, MSAN_ORIGINS and CBM_WORKERS=1 could never change anything;
- with the default slow unwinder (the image's MSan-instrumented
  libunwind) a report exhausted the main thread's 256 MiB stack, and the
  overflow handler then failed with "nested bug in the same thread";
- with fast_unwind_on_fatal=1 the same test prints the
  use-of-uninitialized-value report instead (measured on one tree and
  test: slow unwind aborts at the stack limit, fast unwind reports
  wit/scanner.c:13).

The two reads are fixed in the previous commits of this series, so:

- MSAN_OPTIONS defaults to fast_unwind_on_fatal=1 (the build already
  keeps frame pointers);
- MSAN_EXCLUDE is down to cli, whose install-path failure is a separate,
  still-undiagnosed cause;
- the known-red block, the header (it still claimed CI runs without
  exclusions) and the MSAN_ORIGINS and stack comments say what is true;
- the partial-lane warning and the exclude-name typo check lived in a
  branch behind `elif true` that never ran. They now run in the
  per-suite path, the check as a case match instead of a grep -q pipe
  that pipefail can misreport;
- the comments in scripts/ci/msan-lane.sh and above the test-msan job
  in _test.yml no longer say the five suites overflow.

Signed-off-by: Martin Vogel <martin.vogel.tech@gmail.com>

.github/workflows/_test.yml

@@ -370,16 +370,19 @@ jobs:
   # artifact — the local ladder cannot answer that, having no faithful x86-64
   # emulation. It has now answered it, and the answer SPLIT the list:
   #
-  #   - the five deep-recursion suites overflow their thread stacks on x86-64
-  #     too, so that limit is NOT architectural;
+  #   - five suites aborted with "stack-overflow ... nested bug" on both
+  #     architectures. That was not deep recursion: it was MSan crashing while
+  #     it reported real uninitialized reads (reporting with the slow unwinder
+  #     exhausted the main thread's stack). scripts/msan.sh now unwinds with
+  #     frame pointers, the
+  #     reports print, the reads are fixed, and the five suites run again;
   #   - `cli` does not overflow here at all — it fails in the install path for
   #     an unrelated, still-undiagnosed reason;
   #   - `incremental` was a shadow-memory RSS artifact, now fixed in the test
   #     itself rather than skipped, so it stays in the lane.
   #
   # Per-cause detail lives in scripts/msan.sh, which is the single authoritative
-  # list both venues now share. Zero use-of-uninitialized-value findings were
-  # reported across the lane on either architecture.
+  # list both venues now share.
   test-msan:
     if: ${{ inputs.lanes == 'all' || contains(fromJSON(inputs.lanes), 'msan') }}
     runs-on: ubuntu-latest