fix(ci): honest shard weights + review autofixes

- laneShardFraction: chained --shard invocations sum to full project weight (api backfill was half-weighted)
- exclude *.slow.test.* from duration weighting (engine carried 75 phantom untimed files)
- remove dead ENGINE_PACKAGE_NAME export
- docs: --cold-start-probe, --inputs-dir (snapshot refresh now self-contained), --print-mode

docs/testing.md

@@ -120,6 +120,9 @@ commensurably. Untimed packages are named in a logged warning.
   package duration is apportioned evenly across lanes (logged as `even-apportionment`).
 - **Inspect the plan without running it:** `node scripts/ci-test-shard.mjs --dry-run --total 4`
   (optionally `--shard N`) prints the planned `pnpm` commands and per-shard weight totals.
+- **Measure per-process startup cost:** `node scripts/ci-test-shard.mjs --cold-start-probe <package-name>`
+  runs the package's cheapest test file in isolation and reports `wall − test time` overhead
+  (the signal behind the deferred vitest-4 upgrade gate).
 
 ### Snapshot staleness policy
 
@@ -127,7 +130,9 @@ The snapshot carries `capturedAt`. If it is older than **30 days**, the planner
 prominent warning and proceeds (balance degrades gracefully toward the file-count status
 quo, never below it) — it does **not** fail the build. Refresh is **manual/scheduled from
 the default branch only**: each CI shard uploads per-shard JSON timing artifacts (U1), and
-`node scripts/ci-test-shard.mjs --write-timings` merges them into the snapshot. A future
+`node scripts/ci-test-shard.mjs --write-timings` merges them into the snapshot. Download the
+shard artifacts into `.timings/` first (the default lookup directory), or pass
+`--inputs-dir <path>` to point at wherever they were downloaded. A future
 scheduled job can gate on freshness via `node scripts/ci-test-shard.mjs --check-timings-staleness`,
 which exits non-zero when the snapshot is missing or older than the 30-day budget.
 
@@ -154,6 +159,10 @@ packages affected by your branch diff (plus their reverse-dependents) and skips
 packages whose content hasn't changed since they last passed. A per-package
 pass-cache lives at `node_modules/.cache/fusion/test-cache.json`.
 
+To see which mode a run would pick — and why — without running any tests:
+`node scripts/test-changed.mjs --print-mode` prints the
+`[test-changed] mode=… reason=… packages=…` decision line and exits.
+
 ### What a cache entry's hash covers (dependency-aware invalidation)
 
 Each package's cache hash (`computePackageHash`) folds in, so any of these

scripts/__tests__/ci-test-shard.test.mjs

@@ -29,6 +29,7 @@ import {
   countPackageTestFiles,
   loadPlanningTimings,
   computePackageDurationWeight,
+  laneShardFraction,
   sumFileDurations,
   enumerateDashboardLanes,
   laneProjectNames,
@@ -617,3 +618,39 @@ test("U6: --dry-run prints planned commands and per-shard weight for all 4 shard
   // Dashboard must NOT be virtual-sliced.
   assert.doesNotMatch(result.stdout, /--filter @fusion\/dashboard test --shard/);
 });
+
+test("U6 fix: laneShardFraction sums chained --shard invocations, capped at 1", () => {
+  // Single half-shard lane (app backfill style): genuinely runs 1/4.
+  assert.equal(laneShardFraction("vitest run --project p --shard=1/4"), 0.25);
+  // Chained halves in one lane (api backfill style): runs the FULL project.
+  assert.equal(
+    laneShardFraction("run-heap --shard=1/2 && run-heap --shard=2/2"),
+    1,
+  );
+  // No shard flag: whole project.
+  assert.equal(laneShardFraction("vitest run --project p"), 1);
+  // Over-complete chains clamp at 1.
+  assert.equal(
+    laneShardFraction("a --shard=1/2 && b --shard=2/2 && c --shard=1/2"),
+    1,
+  );
+});
+
+test("U6 fix: computePackageDurationWeight excludes slow-tier files from weighting", (t) => {
+  const projectRoot = mkdtempSync(path.join(tmpdir(), "u6-slow-excl-"));
+  t.after(() => rmSync(projectRoot, { recursive: true, force: true }));
+  mkdirSync(path.join(projectRoot, "packages/eng/src/__tests__"), { recursive: true });
+  writeFileSync(path.join(projectRoot, "packages/eng/src/__tests__/fast.test.ts"), "");
+  writeFileSync(path.join(projectRoot, "packages/eng/src/__tests__/heavy.slow.test.ts"), "");
+  const snapshotPath = writeSnapshot(projectRoot, new Date().toISOString(), {
+    "@x/eng": { files: { "packages/eng/src/__tests__/fast.test.ts": 400 } },
+  });
+  const timings = loadPlanningTimings({ snapshotPath });
+  const weighted = computePackageDurationWeight({ name: "@x/eng", dir: "packages/eng" }, timings, {
+    projectRoot,
+  });
+  // Only the fast file counts: 400ms timed, zero untimed fallback for the
+  // slow file (which the package `test` script never runs).
+  assert.equal(weighted.weight, 400);
+  assert.equal(weighted.partiallyUntimed, false);
+});

scripts/ci-test-shard.mjs

@@ -413,9 +413,6 @@ export const TIMINGS_STALENESS_DAYS = 30;
 /** Dashboard package name; its `test` chain is distributed lane-by-lane. */
 export const DASHBOARD_PACKAGE_NAME = "@fusion/dashboard";
 
-/** Engine package name; kept on `vitest --shard` virtual slicing, by duration. */
-export const ENGINE_PACKAGE_NAME = "@fusion/engine";
-
 /**
  * Load the committed timing snapshot into a flat per-file duration map plus a
  * derived median per-file duration (used to scale the file-count fallback so
@@ -525,7 +522,14 @@ export function sumFileDurations(files, fileDurations) {
  */
 export function computePackageDurationWeight(pkg, timings, options = {}) {
   const projectRoot = options.projectRoot ?? process.cwd();
-  const files = listPackageTestFiles(pkg.dir, { projectRoot }).map((f) => `${pkg.dir}/${f}`);
+  // Exclude `*.slow.test.*` from weighting: the slow tier never runs in a
+  // package's `test` script (it has its own CI gate), so counting those files
+  // — always untimed, hence median-fallback-weighted — inflates the package's
+  // shard weight with work the shard does not execute.
+  const files = listPackageTestFiles(pkg.dir, {
+    projectRoot,
+    extraExclude: (p) => /\.slow\.test\./.test(p),
+  }).map((f) => `${pkg.dir}/${f}`);
 
   const fallbackPerFile = timings.medianPerFileMs > 0 ? timings.medianPerFileMs : DURATION_BUCKET_MS;
   const { durationMs, timedCount, untimedCount } = sumFileDurations(files, timings.fileDurations);
@@ -660,6 +664,26 @@ export function resolveDashboardProjectFiles(dashboardDir, options = {}) {
  * @param {{ projectRoot?: string }} [options]
  * @returns {{ units: Array<{ name: string, lane: string, runKind: "dashboard-lane", weight: number, splittable: false }>, lanes: string[], method: string, untimed: string[] }}
  */
+/**
+ * Fraction of a project a lane command actually runs, derived from its
+ * `--shard=i/n` flags. A lane may chain MULTIPLE --shard invocations of the
+ * same project (e.g. `--shard=1/2 && --shard=2/2` runs the full project):
+ * sum the fractions across all matches, capped at 1. A lane with a single
+ * `--shard=i/n` invocation genuinely runs 1/n. No --shard flag means the
+ * whole project.
+ *
+ * @param {string} command
+ * @returns {number} (0, 1]
+ */
+export function laneShardFraction(command) {
+  const shardMatches = [...command.matchAll(/--shard[=\s]+(\d+)\/(\d+)/g)];
+  if (shardMatches.length === 0) return 1;
+  return Math.min(
+    1,
+    shardMatches.reduce((sum, m) => sum + 1 / Number(m[2]), 0),
+  );
+}
+
 export function buildDashboardLaneUnits(pkg, timings, options = {}) {
   const projectRoot = options.projectRoot ?? process.cwd();
   const pkgJson = JSON.parse(readFileSync(path.join(projectRoot, pkg.dir, "package.json"), "utf8"));
@@ -674,8 +698,7 @@ export function buildDashboardLaneUnits(pkg, timings, options = {}) {
     const units = lanes.map((lane) => {
       const command = scripts[lane] ?? "";
       const projects = laneProjectNames(command);
-      const shardMatch = /--shard[=\s]+(\d+)\/(\d+)/.exec(command);
-      const shardFraction = shardMatch ? 1 / Number(shardMatch[2]) : 1;
+      const shardFraction = laneShardFraction(command);
       const files = new Set();
       for (const project of projects) for (const f of projectFiles[project] ?? []) files.add(f);
       const { durationMs, timedCount, untimedCount } = sumFileDurations([...files], timings.fileDurations);