fix(newos4): rollback keeps scaffolding, checkpoints cannot collide, tests gate the build

Review of the 08:30 implementer run (v4.6.102, checkpoint name fix). The shipped fix was
correct; these are the defects it introduced or left behind, each measured first.

- virtual-fs.js rollback(): the snapshot strips _snapshots/_sessions/.trash for bloat. Restoring
  it verbatim deleted every checkpoint, every agent session and the trash, made rollback
  irreversible and unrepeatable. Re-attach the live copies after the root swap.
- checkpoint(): names are millisecond-resolution, so tight calls overwrote each other
  (measured 6 calls -> 3 snapshots). Dedupe with a -N suffix, refuse names containing "/"
  (written but unlistable), and prune by snapshot createdAt instead of filename sort, which
  let a custom name ("aaa-oldest") evict newer auto-named checkpoints.
- module-predictor.js: legacy pre-v2 points have no `usable`, so a 0.079 s aborted run
  re-entered similarity as evidence and floored every prediction at 1 s. Backfill `usable`
  from the measurement on load, drop the ones it rejects, add a belt-and-braces guard in
  _findSimilar and count only usable points as actual executions.
- newos4-maintain.py: run every node-runnable modules/test/test-*.js on each maintenance pass.
  The v4.6.102 regression was written but no job ever executed it.
- newos4-steer.py: `done` now requires --commit <sha> that exists in the repo, and gains
  `reopen`. A written plan was being closed as done at 06:17 while the predictor fix shipped
  nowhere, so the implementer's pre-flight queue was empty.
- steering.json: predictor item reopened, checkpoint item stamped with f7249ff.
- maintenance board: MAINT-002/005/015 were fixed in v4.6.81-85 but still listed open/high;
  closed with their commit ids, and MAINT-010/016 annotated with the probes that keep them open.

Tests: test-vfs-checkpoint 23/23 (RED 14/9 before), test-predictor-accuracy 38/38
(T1-T3 RED on HEAD), maintain --fix all PASS, browser round-trip
"Checkpoint created: <name>" -> "Rolled back to checkpoint: <name>" on v4.6.103.

Also carries the already-deployed 2026-10-26 syntax-gate hunk in newos4-maintain.py (syntax_failures + gated rebuild) that had never been committed.

1 files changed, 28 insertions, 4 deletions

diff --git a/modules/core/virtual-fs.js b/modules/core/virtual-fs.js
index 09b3548..3ed1b40 100644
--- a/modules/core/virtual-fs.js
+++ b/modules/core/virtual-fs.js
@@ -940,6 +940,17 @@ class VirtualFileSystem {
     }
     checkpoint(name) {
         if (!name) name = new Date().toISOString();
+        name = String(name).trim();
+        if (!name || name.indexOf('/') !== -1) {
+            throw new Error('checkpoint name must be non-empty and must not contain "/"');
+        }
+        // Two unnamed calls can land in the same millisecond; without this the second silently
+        // overwrites the first (measured: 6 calls -> 3 distinct names).
+        if (this.listCheckpoints().indexOf(name) !== -1) {
+            let n = 2;
+            while (this.listCheckpoints().indexOf(name + '-' + n) !== -1) n++;
+            name = name + '-' + n;
+        }
         const createdAt = new Date().toISOString();
         // Strip system dirs to prevent fractal bloat
         const rootClone = JSON.parse(JSON.stringify(this.root));
@@ -959,12 +970,15 @@ class VirtualFileSystem {
         }
         this.fsWriteJSON('/_snapshots/checkpoint-' + name + '.json', data);
         this.saveToStorage();
-        // Auto-manage: keep max 3 checkpoints, delete oldest
+        // Auto-manage: keep the 3 NEWEST checkpoints — by createdAt, not by name, so a custom
+        // name ("aaa-oldest") cannot pin itself forever while newer snapshots are deleted.
         const all = this.listCheckpoints();
         if (all.length > 3) {
-            const sorted = all.slice().sort();
-            const toDelete = sorted.slice(0, all.length - 3);
-            for (const old of toDelete) this.deleteCheckpoint(old);
+            const aged = all.map(n => {
+                const snap = this.fsReadJSON('/_snapshots/checkpoint-' + n + '.json');
+                return { name: n, at: (snap && snap.createdAt) || '' };
+            }).sort((a, b) => (a.at < b.at ? -1 : (a.at > b.at ? 1 : 0)));
+            for (const old of aged.slice(0, aged.length - 3)) this.deleteCheckpoint(old.name);
         }
         return name;
     }
@@ -974,8 +988,18 @@ class VirtualFileSystem {
             console.error('Rollback failed: checkpoint not found or invalid');
             return false;
         }
+        // The snapshot deliberately omits scaffolding (bloat guard at :944-950). Restoring the
+        // clone verbatim deletes every checkpoint, every agent session and the trash, so keep the
+        // live copies and re-attach them after the root swap.
+        const keep = {};
+        for (const dir of ['_snapshots', '_sessions', '.trash']) {
+            const node = this.root && this.root.content ? this.root.content[dir] : null;
+            if (node) keep[dir] = node;
+        }
         this.root = raw.root;
         this.currentPath = raw.currentPath || [];
+        if (!this.root.content) this.root.content = {};
+        for (const dir of Object.keys(keep)) this.root.content[dir] = keep[dir];
         this.saveToStorage();
         return true;
     }