Fix source-safety, detector, and lock defects in Live polling work

Addresses the review findings on #371, plus several the bots did not catch.
All fixes have regression coverage that fails on the prior code.

Source corruption:
- Vue accept dropped valueless root attrs (disabled, v-cloak) and, worse,
  rewrote @click="x" as a literal click="x" DOM attribute, because the attr
  parser was name-anchored and skipped the sigil. Tokenize the whole Vue attr
  grammar and normalize shorthands so accept round-trips directives.
- --variant was interpolated unescaped into a RegExp, so --variant '.*' matched
  the original block first and reported a successful accept while silently
  restoring the original. Validate against the digits pattern the browser and
  the /events schema already enforce.
- --id reached path.join unvalidated, so --id ../../../../etc/evil wrote and
  read receipts outside the project. Hoist the existing safeSessionId check
  into impeccable-paths and apply it at every id-to-path sink.

Accept/lock correctness:
- Plain HTML/JSX accept and discard did not catch SOURCE_LOCKED, so contention
  exited non-zero with empty stdout and the agent got no JSON to retry on.
- Lock staleness was mtime-only and never read the pid it records: a holder
  whose critical section outran 60s had its live lock swept, admitting a second
  writer to the same file, while a crashed holder blocked accepts for a full
  60s. Decide staleness by owner liveness, and release only our own lock.

Detector:
- isNeutralColor only parses computed color forms, so routing authored CSS
  through it reported inset 4px 0 0 #000 / black / #e5e7eb as chromatic
  side-tab stripes. Add an authored-color neutrality test covering hex and
  named neutrals; the fixture had no literal-color cases at all.
- Rule line numbers were off by one for every rule after the first, and
  commented-out CSS was scanned as live rules.

Server:
- An error reply carries no sourceEventType, and inferSourceEventType returned
  undefined, which acknowledgePendingEvent treats as a wildcard: a stale
  generate worker's failure consumed the user's queued Accept, which then
  reached no agent and left the browser in SAVING forever.
- The generate preflight spawned live-wrap.mjs synchronously inside the request
  handler, freezing the single-threaded server for the whole scaffold (~7.6s
  measured on this repo, 15s ceiling) and stalling Accept/Discard/SSE. Make it
  async, claiming the lease before the first await so no event double-delivers.
- Every browser checkpoint was echoed back as variant_progress, so a Tune
  slider drag remounted the preview under the user's cursor and latched the
  *_reviewable phases from the wrong trigger. Gate on the reason.

Cleanup:
- Collapse four divergent benchmark argv parsers into scripts/lib/cli-args.mjs.
  Three silently misread flags: --iterations 20 benchmarked 5, --agent llm ran
  the fake agent, --median-target=0.4 used the default threshold.
- Drop a snapshot cache this branch made write-only (it grew per session for
  the server's lifetime and was never read), a dead exported reconcile helper,
  and the unused deferReply branch.

Prepared with AI assistance under maintainer direction.

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

skill/scripts/live-accept.mjs

@@ -16,7 +16,7 @@
 import fs from 'node:fs';
 import path from 'node:path';
 import { isGeneratedFile } from './lib/is-generated.mjs';
-import { getLiveDir } from './lib/impeccable-paths.mjs';
+import { getLiveDir, safeSessionId } from './lib/impeccable-paths.mjs';
 import { readBuffer as readManualEditsBuffer, writeBuffer as writeManualEditsBuffer } from './live/manual-edits-buffer.mjs';
 import { withSourceLockSync } from './live/source-lock.mjs';
 import {
@@ -37,6 +37,9 @@ import {
 
 const EXTENSIONS = ['.html', '.jsx', '.tsx', '.vue', '.svelte', '.astro'];
 const ACCEPT_LOCK_WAIT_MS = 1_000;
+// Mirrors VARIANT_ID_PATTERN in live/event-validation.mjs, which gates the same
+// value arriving over HTTP.
+const VARIANT_NUM_PATTERN = /^[0-9]{1,3}$/;
 
 // ---------------------------------------------------------------------------
 // CLI
@@ -75,7 +78,19 @@ Output (JSON):
   const isDiscard = args.includes('--discard');
 
   if (!id) { console.error('Missing --id'); process.exit(1); }
+  // `id` becomes a path segment (accept receipts, preview manifests, generated
+  // component dirs). Reject separators and traversal here so one check covers
+  // every downstream sink.
+  try { safeSessionId(id); } catch { console.error('Invalid --id'); process.exit(1); }
   if (!isDiscard && !variantNum) { console.error('Need --discard or --variant N'); process.exit(1); }
+  // `variantNum` is interpolated into a RegExp and into the markup written back
+  // to source. The browser and the /events schema both constrain it to digits;
+  // enforce the same here, or `--variant '.*'` matches the `original` block
+  // first and silently accepts the original while reporting success.
+  if (!isDiscard && !VARIANT_NUM_PATTERN.test(variantNum)) {
+    console.error('Invalid --variant');
+    process.exit(1);
+  }
 
   const requestedOperation = isDiscard ? 'discard' : 'accept';
   const priorReceipt = readAcceptReceipt(process.cwd(), id);
@@ -296,10 +311,25 @@ Output (JSON):
   }
 
   if (isDiscard) {
-    const result = handleDiscard(id, lines, targetFile);
+    let result;
+    // handleDiscard takes the source lock, which throws SOURCE_LOCKED under
+    // contention. Without this catch the CLI exits non-zero with empty stdout
+    // and the agent gets no JSON to act on.
+    try {
+      result = handleDiscard(id, lines, targetFile);
+    } catch (err) {
+      emitResult({ handled: false, file: relFile, error: err.message });
+      return;
+    }
     emitResult({ handled: true, file: relFile, carbonize: false, ...result });
   } else {
-    const result = handleAccept(id, variantNum, lines, targetFile, paramValues);
+    let result;
+    try {
+      result = handleAccept(id, variantNum, lines, targetFile, paramValues);
+    } catch (err) {
+      emitResult({ handled: false, file: relFile, error: err.message });
+      return;
+    }
     const acceptedOriginalText = result.acceptedOriginalText || '';
     delete result.acceptedOriginalText;
     // Single-line attention-grabber when cleanup is required. The full
@@ -1003,7 +1033,7 @@ function searchDir(dir, query, seen, depth) {
 // ---------------------------------------------------------------------------
 
 function acceptReceiptPath(cwd, id) {
-  return path.join(getLiveDir(cwd), 'accept-receipts', `${id}.json`);
+  return path.join(getLiveDir(cwd), 'accept-receipts', `${safeSessionId(id)}.json`);
 }
 
 function readAcceptReceipt(cwd, id) {