ci(shard-timings): judge each refresh candidate at its own shard width

The 6 -> 8 resize made select-shard-timings-run reject every recent
schedule run (all 6-shard, all whole and green) as missing Test Core
7/8, 8/8, so the refresh exited 1 on this PR (job 121075952437) and
would have kept doing so until an 8-shard schedule run existed with its
artifacts retained.

The dataset is per package; the width a run was split at is not an
input to any package's weight. runIsEligible now reads the run's width
from its own `Test Core (N/M)` job names and admits it when all M shards
concluded success with all M summaries retained. A run whose Test Core
jobs disagree on M, or that has none, is refused by name. The download
loop already iterates each candidate's own artifact_ids, so a resize
has no transition window.

New self-test battery (width change, 9 cases) and an accumulation leg
mixing 8-wide and 6-wide runs, both confirmed to fail against the old
fixed-width reading.

Co-authored-by: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01M781cxsyRAqT49LvLkJKXb

Author: claude

.github/workflows/shard-timings-refresh.yml

@@ -47,7 +47,9 @@
 # HOW IT GETS ITS INPUTS, AND WHY IT RUNS HERE RATHER THAN IN AN AGENT WORKTREE
 # ----------------------------------------------------------------------------
 # The documented refresh path downloads the `test-core-run-summary-N-of-M`
-# artifacts of a green run, one per Test Core shard (M = SHARD_COUNT).
+# artifacts of a green run, one per Test Core shard. M is the RUN's own width,
+# read from its `Test Core (N/M)` job names, so runs from before and after a
+# matrix resize both read (#25376).
 # `GET /actions/artifacts/{id}/zip` redirects to
 # `productionresultssa*.blob.core.windows.net`, and every agent container's
 # egress policy answers 403 to CONNECT for that host — reproduced four ways, on
@@ -484,9 +486,10 @@ jobs:
 
             OK=1
             # One summary per shard the selector found: it admits a run only when
-            # every one of SHARD_COUNT (select-shard-timings-run.mjs) shards
-            # concluded success with its artifact retained, so these keys ARE the
-            # roster. A count spelled here would be a third copy of it (#25376).
+            # every shard OF THAT RUN concluded success with its artifact
+            # retained, at the run's own width (select-shard-timings-run.mjs,
+            # runShardWidth), so these keys ARE that run's roster. A count
+            # spelled here would break the first refresh after a resize (#25376).
             SHARD_NS=$(node -e 'const r=JSON.parse(require("fs").readFileSync(process.env.RUNNER_TEMP+"/candidates.json","utf8"))[Number(process.argv[1])];console.log(Object.keys(r.artifact_ids).map(Number).sort((a,b)=>a-b).join(" "))' "$i")
             for N in $SHARD_NS; do
               ART=$(node -e 'const r=JSON.parse(require("fs").readFileSync(process.env.RUNNER_TEMP+"/candidates.json","utf8"))[Number(process.argv[1])];console.log(r.artifact_ids[process.argv[2]])' "$i" "$N")