chore: release signage 0.1.2

0.1.2 fixes what the first production Signage deployment found:
- #4: the display and server share one message schema; every heartbeat
  from an image slide used to be refused with a 400;
- #5: every display request retries with backoff and never gives up, so
  a display that failed one request no longer stays offline until
  reloaded;
- #6: one object URL per cached slide, released when done; the leak
  exhausted the browser within minutes;
- CI (#7, #8): every change runs inside a pinned Ekklesia, end to end
  in a real browser, and cleans up even when killed.

compatibility.platform stays ^0.1.0: every fix is module-side, and CI
tested it against platform v0.1.1 plus test-only scripts. The README no
longer says Signage is unreleased or undeployed.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@ekklesia/signage",
-  "version": "0.1.1",
+  "version": "0.1.2",
   "private": true,
   "type": "module",
   "description": "Ekklesia's visual programming, timeline, synchronization, and display-delivery module.",