feat: check the run's preconditions, and record which tuner cache it used

Two failure modes that cost this harness real results, both of them silent.

A wrong path does not fail as a wrong path. pytest cannot collect a benchmark
directory that is not there, so benchmark/conftest.py never registers --level
and --mode, and the run dies with "unrecognized arguments: --level core --mode
kernel" -- an error about the flags, several layers away from the missing
directory. run_pytest.sh now checks the FlagGems directory, the benchmark file,
SHAPE_FILE and REPLAY_FROM before starting pytest, prints
"PREFLIGHT_FAILED check=<the named check>" and exits 3, distinct from pytest's
own 1 and 4 so the batch layer can tell a broken environment from a broken
operator. Only the file half of a node is checked; a missing ::test_<fn> already
produces a clear "not found: ..." from pytest and needs no help.

FlagGems keeps libtuner winners in a sqlite DB under $FLAGGEMS_CACHE_DIR
(default ~/.flaggems), shared by every run on the machine. Whichever side of an
A/B runs first therefore hands its winners to the second, which then compiles
only those: measured on router_gemm, the second of two shared-cache runs
compiled 10 variants instead of 234 and finished in 17s instead of 52s. That is
how an mm A/B in this repo produced "945 variants -> 33, 136 converts -> 10" and
read as a compiler result when it was cache inheritance; isolated, both runs
compile 234.

ZL_ISOLATE_TUNER_CACHE=1 gives a run its own cache directory, dropped afterwards
with the Triton one since the chosen configs are already in autotune_records/.
It defaults to 0, because cold tuning is expensive and the standard REPLAY_FROM
A/B pins configs anyway; only record-against-record is exposed. The default is
unchanged, but the caliber is no longer invisible: either way it goes into the
run header and run_identity.json as tuner_cache, which is what was missing when
the mm numbers could not be explained after the fact.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

_run_identity.py

@@ -71,6 +71,10 @@ def main(argv=None) -> int:
     p.add_argument("--test-file", default=os.environ.get("TEST_FILE", ""))
     p.add_argument("--shape-mode", default=os.environ.get("SHAPE_MODE", ""))
     p.add_argument("--bench-level", default=os.environ.get("BENCH_LEVEL", "core"))
+    # "isolated" or "shared(<path>)". Recorded because a shared libtuner cache
+    # lets whichever side runs first hand its winners to the other, and after
+    # the fact the two runs are otherwise indistinguishable.
+    p.add_argument("--tuner-cache", default="")
     args = p.parse_args(argv)
     init_identity(
         args.out_dir,
@@ -83,6 +87,7 @@ def main(argv=None) -> int:
         test_file=args.test_file,
         shape_mode=args.shape_mode,
         bench_level=args.bench_level,
+        tuner_cache=args.tuner_cache,
     )
     return 0