fix(#4434): win32 unmeasured test files weigh the documented ~2.2x Windows-cost floor, not the Linux-measured mean (#4903)

* fix(#4434): win32 unmeasured test files weigh the documented ~2.2x Windows-cost floor, not the Linux-measured mean

test(#4434): failing-first — win32 unmeasured-file weight must use the
documented Windows-cost multiplier, not the plain table mean

fix(#4434): makeFileWeigher takes a platform and applies
WINDOWS_UNMEASURED_COST_MULTIPLIER (2.2) to the unmeasured-file fallback
on win32 only; measured files and other platforms are unaffected

chore(#4434): changeset fragment

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

* test(#4434): add fast-check property coverage for the win32 unmeasured-file weight multiplier

CLAUDE.md requires a fast-check property test for budget-limit logic; the
prior example-based #4434 tests didn't satisfy that. Adds two seeded,
bounded property tests: the unmeasured-file fallback matches the platform
rule for any measured table, and a measured file's weight is
platform-invariant for any measured table.

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

* chore(#4434): backfill changeset PR number (4903)

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

* fix(#4434): the Windows unmeasured-file multiplier must not apply when there is no timings table at all

Real Windows CI on this PR caught a regression my own linux-only gsd-test
verification couldn't see: makeFileWeigher applied
WINDOWS_UNMEASURED_COST_MULTIPLIER even when `timings` is null (missing/
corrupt/empty table), breaking the pre-#2456 "no table degrades to uniform
weight 1" invariant several existing tests depend on. The multiplier now
only applies to a file absent from an otherwise-loaded table — the actual
#4434 mechanism (a real, loaded, Linux-measured table with unmeasured
entries) — never to the no-table-at-all path.

Six pre-existing tests that called makeFileWeigher/pack helpers with no
explicit platform (silently inheriting whatever OS runs them) now pin an
explicit 'linux' platform, since they test the platform-agnostic mean-vs-
median and Object.prototype-safety invariants, not #4434's Windows
behavior. One subprocess-level test is isolated from the real committed
tests/test-timings.json via RUN_TESTS_TIMINGS_FILE, matching this file's
existing isolation convention for cost-profile-sensitive assertions.

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

---------

Co-authored-by: sim <sim@local>
Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

scripts/run-tests.cjs

@@ -535,12 +535,36 @@ function loadTestTimings(timingsPath) {
   return { timings, mean, medianWeight: median / mean };
 }
 
+// #4434: the table's own `sources` are Linux-only (test-events-linux-node22/24
+// .jsonl — never a Windows event stream), and this runner's own chunk-kill
+// diagnostic already tells operators "real Windows cost runs ~2.2x the
+// recorded figure, so treat every number as a floor" (see the catch block
+// around the per-chunk timeout, below). That string was, until this fix,
+// advisory text ONLY — nothing in the weigher actually applied it. Verified
+// live: `next` @ ccfed6335 (unrelated to the chunk's own diff — see #4434)
+// killed Windows conformance shard 3/3 chunk 6/8 at 600016ms; 6 of that
+// chunk's 17 files were wholly absent from the table and were packed at the
+// table's plain mean, identically to how a Linux/macOS chunk would price
+// them. A MEASURED file does not get this multiplier: #4733 already
+// calibrates measured-file packing against a real Windows wall-clock via
+// MAX_FILES_PER_CHUNK, so inflating measured weights again here would
+// double-apply the correction. An unmeasured file has no real data at all —
+// it is exactly the "floor, not a verdict" case the diagnostic already warns
+// about, so only its fallback gets the multiplier, and only on win32.
+const WINDOWS_UNMEASURED_COST_MULTIPLIER = 2.2;
+
 // Build the packer's weight function from a loaded timing table.
 //
 // A file present in the table weighs its measured duration relative to the
 // table mean. A file ABSENT from it weighs 1 — the table MEAN, the same
 // value a null table (missing or unparseable file) yields for every file,
-// because both states mean the same thing: cost unknown.
+// because both states mean the same thing: cost unknown. On win32 the
+// fallback is WINDOWS_UNMEASURED_COST_MULTIPLIER instead of 1 (see #4434,
+// above) — but ONLY for a file absent from an otherwise-loaded table; a
+// completely missing/corrupt/empty timings file (`timings` is `null`) still
+// degrades to uniform weight 1 on every platform, matching the pre-#2456
+// count-based-packing invariant many existing tests depend on. Every other
+// platform keeps the plain mean.
 //
 // This was previously `timings.medianWeight`, on the claim that an absent
 // file "costs chunk balance, never a red build." That claim is false. In a
@@ -550,11 +574,14 @@ function loadTestTimings(timingsPath) {
 // cause a red build: Windows conformance shard 2/3, chunk 4/6 was killed at
 // 600018ms with ZERO failing tests, because files absent from the table
 // packed as if they were nearly free and the chunk blew the 600s cap.
-// Empirically, mean is the right estimate for an unknown file: 9 unmeasured
-// files that caused the incident averaged 6659ms against a table mean of
-// 7152ms — within 7%.
-function makeFileWeigher(timings) {
+// Empirically, mean is the right estimate for an unknown file on the
+// platform the table was MEASURED on: 9 unmeasured files that caused the
+// incident averaged 6659ms against a table mean of 7152ms — within 7%. That
+// equivalence does not hold on win32, where the table's own sources are
+// Linux-only (#4434).
+function makeFileWeigher(timings, platform = process.platform) {
   if (!timings) return () => 1;
+  const unmeasuredWeight = platform === 'win32' ? WINDOWS_UNMEASURED_COST_MULTIPLIER : 1;
   return (f) => {
     const key = basename(f);
     // Own-property check before the lookup. This is defense-in-depth, NOT a
@@ -567,7 +594,9 @@ function makeFileWeigher(timings) {
     // keeps the lookup correct for arbitrary input, since this function is
     // exported and does not control its caller's strings.
     const ms = Object.hasOwn(timings.timings, key) ? timings.timings[key] : undefined;
-    return typeof ms === 'number' && Number.isFinite(ms) && ms >= 0 ? ms / timings.mean : 1;
+    return typeof ms === 'number' && Number.isFinite(ms) && ms >= 0
+      ? ms / timings.mean
+      : unmeasuredWeight;
   };
 }
 
@@ -1192,7 +1221,7 @@ function main() {
   let weigherMemo = null;
   const fileWeightOf = () => {
     if (weigherMemo === null) {
-      weigherMemo = makeFileWeigher(loadedTimings());
+      weigherMemo = makeFileWeigher(loadedTimings(), process.platform);
     }
     return weigherMemo;
   };
@@ -1854,6 +1883,7 @@ module.exports = {
   defaultMaxFilesPerChunk,
   loadTestTimings,
   makeFileWeigher,
+  WINDOWS_UNMEASURED_COST_MULTIPLIER,
   makeMeasuredPredicate,
   packChunks,
   // 2026-09-07 (PR #4497): the codex-config.test.cjs chunk-isolation fix —