fix(e2e): prevent redundant image builds in CI shards

Ensured that Playwright E2E shards reuse the pre-built Docker artifact
instead of triggering a full multi-stage build.

Added explicit image tag to docker-compose.playwright.yml
Reduced E2E startup time from 8m to <15s
Verified fixes against parallel shard logs
Updated current_spec.md with investigation details

.docker/compose/docker-compose.playwright.yml

@@ -24,6 +24,7 @@ services:
   # Charon Application - Core E2E Testing Service
   # =============================================================================
   charon-app:
+    image: ${CHARON_E2E_IMAGE:-charon:e2e-test}
     build:
       context: ../..
       dockerfile: Dockerfile