* perf(pico): implement O(1) session lookup for pico connections
- Replace `sync.Map` with `connections` and `sessionConnections`.
- Add `addConnection`, `removeConnection`, `sessionConnectionsSnapshot`, and `takeAllConnections` with `connsMu` for concurrency.
- `broadcastToSession` now dispatches directly to `sessionConnections`.
- Add `newUniqueConnID` to avoid UUID collision/overwrites.
- Ensure `Stop` and `readLoop` use the new helpers for safe cleanup and correct `connCount` updates.
* refactor(pico): replace addConnection with createAndAddConnection for atomic connID generation
* refactor(pico): clear connections in one time to improve perf
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
* fix(pico): keep connCount consistent with connection indexes
* refactor(pico): make connCount a regular int guarded by connsMu
* fix(pico): enforce MaxConnections atomically on registration
* fix(pico): use temporary over-limit error and remove conn counter
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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
