Step 10j (host + extension): cache contract three-state UX

Completes the two-tier cache contract from step 9 / docs/CACHE_CONTRACT.md
on the extension side. The Python side shipped in the Python repo at
33c495a.

Host (rcjav-host.py):
  - fetch_rules_info() memoizes per-script-path calls to
    `rc-jav.py --print-rules-info` so handle_cache_status doesn't pay
    the Python startup cost on every poll.
  - _cache_freshness_fields(data, rules_info) computes the new
    cache_schema / id_rules / id_rules_signature trio + their three
    *_match booleans + cache_state ('fresh' / 'stale_by_rules' /
    'schema_mismatch' / 'missing'). Legacy version:3 caches still on
    disk report as stale_by_rules with cache_schema_match=True (we'll
    migrate them at next load_cache).
  - New handle_reextract_ids() action forwards to
    `rc-jav.py --reextract --format json` with a 5-minute timeout.

background.js:
  - New `reextract-ids` message forwards to host with a 300s timeout.

options-cache.js + options-library-issues.js:
  - renderCacheContractBanner() paints a three-state banner above the
    per-remote list: green ✓ fresh / amber ! stale-by-rules (with
    "Re-extract IDs (fast, no rescan)" chip button) / red ✗ schema
    mismatch. Includes a snippet of the cache signature for diagnostics.
  - Delegated click handler in options-library-issues.js catches
    .cache-reextract, sends the message, shows transient
    "Re-extracting…" state, and replaces the button with a per-summary
    line ("Re-extracted N IDs · X changed · Y unchanged · Z dropped").
  - rules_info_error from the host surfaces as its own amber line above
    the banner.

node --check passes on background.js, options-cache.js,
options-library-issues.js individually and on the concatenation of all
four script files. python -m py_compile passes on rcjav-host.py.
Behavioral verification requires reloading the unpacked extension and
running through:
  - Check Cache → banner shows "stale by rules" amber (legacy v3 cache)
  - Click "Re-extract IDs" → fast path runs, summary appears
  - Check Cache again → banner now shows "Cache up to date" green

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

background.js

@@ -948,6 +948,19 @@ chrome.runtime.onMessage.addListener((msg, sender, sendResponse) => {
     })();
     return true;
   }
+  if (msg.type === "reextract-ids") {
+    (async () => {
+      try {
+        const settings = await getSettings();
+        const r = await nativeCall({
+          action: "reextract_ids",
+          rcjav_path: settings.rcjavPath || "",
+        }, 300_000);
+        sendResponse(r);
+      } catch (e) { sendResponse({ ok: false, error: e.message, error_kind: classifyNativeError(e), extension_id: chrome.runtime.id }); }
+    })();
+    return true;
+  }
   if (msg.type === "host-status") {
     (async () => {
       try {