Prevent conversation visibility races during organisation switching

[witbrock]

Outcome

Prevents content and metadata from the previously selected organisation from remaining visible or being repainted by late asynchronous responses while an organisation switch is in progress.

Changes

• emit an organisation-switch-start event synchronously and serialise organisation mutations
• clear conversation-derived UI and caches at switch start
• fence history, session-list, invite, focus, links, queue, new-chat, and live-response work by organisation generation
• accept only the latest switch completion and recover the canonical context after a failed switch
• add regression coverage for late old-organisation responses and rapid/superseded switches

Validation

• 12 targeted race tests pass
• neighbouring chat-tab suite: 272 tests pass
• 59 neighbouring organisation/context tests pass
• JavaScript syntax checks and frontend static lint pass
• candidate server starts with the configured environment; /health reports Mongo connected and model registry ready
• live /von/ page loads in headless Chrome without page errors

The broader frontend run has 9 pre-existing failures in 4 unrelated/independently reproducible suites; no acceptance claim is made for those paths.