feat(career): profile passport wall, home career card, shareable PNG card (#955)

* feat(career): profile passport wall, home career card, shareable PNG card

Career v3, WS2. The identity artifact leaves the plugin tab:

- Profile: #v3-profile-passports-mount (core, one div) filled by career
  on v3:profile-rendered — per-instrument shelves of earned covers,
  hours, gig count, open-career link. Absent-not-empty.
- Home: the plugin-count stat tile becomes #v3-dash-career-slot with the
  old stat as fallback content; career replaces it with a trading-card
  tile (leather + foil shine, badge count, hours, closest-stamp ask) on
  the existing v3:dashboard-rendered event.
- Shareable card: static/js/blob-io.js (downloadBlob lifts the idiom
  duplicated verbatim in settings-io/diagnostics-export — both
  refactored; copyImageBlob wraps ClipboardItem, returns false to signal
  the download fallback). Earned passports get Save/Copy card: a
  natively-drawn 480×640 canvas (leather, stamp ring, stubs+hours line);
  copy falls back to download with a notice when the clipboard refuses.
- Mount-point convention documented in docs/plugin-v3-ui.md.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(career): external surfaces stay absent until a passport exists

CodeRabbit on #955: a bare commitment produced a zero-passport wall and
replaced the dashboard fallback. Docs also now say mounts may hold
fallback content and plugins REPLACE, never append.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

static/js/blob-io.js

@@ -0,0 +1,28 @@
+// Blob export helpers — the download idiom that used to be duplicated in
+// settings-io.js and diagnostics-export.js, plus image-to-clipboard for
+// shareable cards/posters. A LEAF module: imports nothing. Classic-script
+// plugins reach it via dynamic import('/static/js/blob-io.js').
+
+export function downloadBlob(blob, filename) {
+    const url = URL.createObjectURL(blob);
+    const a = document.createElement('a');
+    a.href = url;
+    a.download = filename;
+    document.body.appendChild(a);
+    a.click();
+    document.body.removeChild(a);
+    URL.revokeObjectURL(url);
+}
+
+// Copy an image blob to the system clipboard. Returns true on success, false
+// when the Clipboard API is unavailable or refuses (insecure context, no user
+// gesture, permission denied) — callers fall back to downloadBlob and say so.
+export async function copyImageBlob(blob) {
+    try {
+        if (!navigator.clipboard || typeof ClipboardItem === 'undefined') return false;
+        await navigator.clipboard.write([new ClipboardItem({ [blob.type || 'image/png']: blob })]);
+        return true;
+    } catch (_) {
+        return false;
+    }
+}