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>

options-cache.js

@@ -107,6 +107,35 @@ document.getElementById("setup-health-run").addEventListener("click", (event) =>
   })
 );
 
+// Three-state UX (docs/CACHE_CONTRACT.md): fresh / stale_by_rules / schema_mismatch.
+// Renders an inline banner above the per-remote list. Stale_by_rules adds a
+// "Re-extract IDs" button that triggers the fast rebuild without rclone.
+function renderCacheContractBanner(r) {
+  const state = r.cache_state;
+  if (r.rules_info_error) {
+    return `<div style="margin-top:10px;padding:6px 8px;background:rgba(255,200,50,.08);border:1px solid rgba(255,200,50,.25);border-radius:4px;color:#ffa;">⚠ rules lookup failed: ${escapeHtml(r.rules_info_error)}</div>`;
+  }
+  if (state === "fresh") {
+    return `<div style="margin-top:10px;padding:6px 8px;background:rgba(120,200,120,.08);border:1px solid rgba(120,200,120,.25);border-radius:4px;color:#afa;">✓ Cache up to date with current ID rules.</div>`;
+  }
+  if (state === "stale_by_rules") {
+    const sigLine = r.id_rules_signature && r.id_rules_signature !== "legacy"
+      ? `<div style="color:#999;font-size:11px;margin-top:3px;">Cache signature: <code>${escapeHtml(String(r.id_rules_signature).slice(0, 22))}…</code></div>`
+      : `<div style="color:#999;font-size:11px;margin-top:3px;">Cache predates the two-tier contract (legacy header).</div>`;
+    return `<div style="margin-top:10px;padding:8px 10px;background:rgba(255,200,50,.08);border:1px solid rgba(255,200,50,.3);border-radius:4px;color:#ffa;">
+      ! <strong>Cache is stale by rules.</strong> ID extraction rules have changed since this cache was built. Some <code>jav_id</code> values may be out of date.
+      ${sigLine}
+      <div style="margin-top:8px;"><button class="chip-btn cache-reextract" type="button" style="color:#ffd97a;background:rgba(255,200,50,.12);border-color:rgba(255,200,50,.35);font-weight:600;">Re-extract IDs (fast, no rescan)</button></div>
+    </div>`;
+  }
+  if (state === "schema_mismatch") {
+    return `<div style="margin-top:10px;padding:8px 10px;background:rgba(255,120,120,.08);border:1px solid rgba(255,120,120,.3);border-radius:4px;color:#faa;">
+      ✗ <strong>Cache schema mismatch.</strong> The on-disk cache shape is incompatible (schema ${escapeHtml(r.cache_schema ?? "?")} vs expected ${escapeHtml(r.expected_cache_schema ?? "?")}). A full re-scan is required.
+    </div>`;
+  }
+  return "";
+}
+
 document.getElementById("cache-status-run").addEventListener("click", async () => {
   const out = document.getElementById("cache-status-results");
   out.textContent = "checking cache...";
@@ -136,6 +165,7 @@ document.getElementById("cache-status-run").addEventListener("click", async () =
       `<div><span style="color:#777;">Configured target:</span> ${escapeHtml((r.configured?.default_target || []).join(", ") || "(none)")}</div>`,
       `<div><span style="color:#777;">Configured source:</span> ${escapeHtml((r.configured?.default_source || []).join(", ") || "(none)")}</div>`,
     ];
+    rows.push(renderCacheContractBanner(r));
     for (const m of r.remotes || []) {
       const color = m.status === "never_scanned" || m.stale ? "#ffa" : "#afa";
       const state = m.status === "never_scanned" ? "never scanned" : `${m.status || (m.stale ? "stale" : "fresh")} · age ${fmtCacheAge(m.age_hours)}`;