fix(msan): shard the lane one suite per process, origins default 1

With the zstd feature-macro fix in, the x86-64 leg finally RUNS -- and
thousands of tests pass before the known deep-recursion stack overflow
lands in grammar_regression, the same signature as the local arm64 wall.
So it was never an aarch64 shadow-mapping artifact: it is origin-tracking
frame inflation meeting the deepest parser recursion in the tree.

The lane's own recorded analysis (item 4) showed the wall MOVES with
cumulative process state -- thread ordinals were in the hundreds by the
time the deep suites ran, and the same suites at the same flags behaved
differently by run context. A fresh process per suite removes that axis
while keeping COMPLETE coverage: every suite still runs, none excluded,
which is the line this lane refuses to cross. Suite enumeration comes
from --list-suites, whose completeness the sharding union guard already
proves.

Origins drop to 1 by default on the lane: detection is IDENTICAL at
every origin level -- only report depth differs -- and the frame savings
are what lets the deep suites fit their stacks. MSAN_ORIGINS=2 remains a
local override for chasing a specific report's origin chain.

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

scripts/msan.sh

@@ -35,7 +35,11 @@ fi
 # 1 = immediate origin only, 0 = none. Detection is IDENTICAL at every level;
 # only report quality differs, so lowering it is the first lever to try when
 # frame inflation overflows a deep-recursion suite.
-MSAN_ORIGINS="${MSAN_ORIGINS:-2}"
+# Default 1 on this lane: identical DETECTION at every level (only report
+# depth differs), and the frame savings are what lets the deep-recursion
+# grammar suites fit their stacks under instrumentation. Export
+# MSAN_ORIGINS=2 locally when chasing a specific report's origin chain.
+MSAN_ORIGINS="${MSAN_ORIGINS:-1}"
 if [ "$MSAN_ORIGINS" = "0" ]; then
     MSAN_ORIGIN_FLAG=""
 else
@@ -130,7 +134,21 @@ MSAN_EXCLUDE="${MSAN_EXCLUDE:-}"
 if [ "$#" -gt 0 ]; then
     ./build/msan/test-runner "$@"
 elif [ -z "$MSAN_EXCLUDE" ]; then
-    ./build/msan/test-runner
+    # One suite per process. Item 4 above proved the overflow wall moves with
+    # CUMULATIVE process state (thread ordinals in the hundreds by the time
+    # the deep suites run), so a fresh process per suite removes that axis
+    # while keeping COMPLETE coverage — every suite still runs, uninstrumented
+    # none. The union of --list-suites is proven complete by the sharding
+    # guard from the per-leg parallelism work (#1164).
+    fails=""
+    for suite in $(./build/msan/test-runner --list-suites); do
+        echo "=== msan: $suite ==="
+        ./build/msan/test-runner "$suite" || fails="$fails $suite"
+    done
+    if [ -n "$fails" ]; then
+        echo "=== MSan lane FAILED suites:$fails ===" >&2
+        exit 1
+    fi
 else
     echo "WARNING: running a PARTIAL sanitizer lane — excluded: $MSAN_EXCLUDE" >&2
     echo "WARNING: a green result here does NOT mean the tree is MSan-clean." >&2