test: add E2E coverage for memory sync pipeline + graph visualization

[senamakel]

Summary

PR #3113 introduced significant new functionality that needs E2E test coverage:

Core (Rust) — integration tests needed

• memory_sync::sources::github::run_github_sync — mock GitHub API, verify summaries land in tree
• memory_tree::ingest::ingest_summary — verify L1 buffer management, seal cascade at SUMMARY_FANOUT
• memory_sync::sources::rebuild::rebuild_tree_from_raw — seed raw files on disk, verify tree gets built
• memory_sources::sync::sync_source — verify per-source mutex (second call is no-op), retry_all_failed fires, audit log written
• memory_sources::sync::check_and_rebuild_tree — verify auto-detect triggers rebuild when raw exists + max_level=0
• Graph export with source root nodes — verify synthetic roots, document leaves, orphan linking

Frontend (Vitest) — component tests needed

• SyncAuditPanel — renders entries, formats tokens/cost/duration, handles empty state
• PixiGraph.updateGraph — verify nodes preserve position on update, new nodes spawn near parent
• MemoryWorkspace — verify graph doesn't unmount between fetches (no setGraph(null))
• MemoryGraph — source node kind renders with orange color, legend includes "Source" entry

E2E (WDIO/Playwright)

• Memory tab → trigger GitHub sync → verify graph shows source root + L1/L2 nodes
• Memory tab → trigger Gmail sync → verify audit log panel shows the run
• Memory tab → press sync twice quickly → verify only one sync runs (mutex)

Context

These were validated manually during #3113 development but lack automated coverage. The coverage gate (≥80% on changed lines) will benefit from these tests on future PRs touching the same paths.