fix(chat): open notification targets and allow memory overlap

[danvitv]

Summary

• route notification taps through the app router so the intended chat, session, and message open
• allow chat and Memory Book draft generation to overlap while retaining per-draft and memory-workflow ownership
• add production-level concurrency tests proving reversed completions persist to their own stores
• stabilize Windows clipboard image tests by waiting for pasteboard file I/O
• update generation invariants and architecture documentation

Memory concurrency note

The previous chat-memory mutex was an intentional safety policy, not an accidental implementation bug. Current transports, callbacks, cancellation ownership, and persistence paths are isolated, so this PR relaxes that policy and adds marker-based regression coverage. If real providers or proxies show cross-request contamination, the conservative mutex can be restored.

Verification

• flutter test: 3870 passed
• targeted concurrency, notification, dispatcher, memory lifecycle, and clipboard suites passed
• focused dart analyze: clean
• full flutter analyze: no new issues; 9 pre-existing unrelated findings remain
• git diff --check: clean