fix: speaker assignment not saving when selected from dropdown

[devin-ai-integration]

Summary

Fixes #4860. Two bugs in the Rust transcript rendering pipeline prevented user-selected speaker assignments from taking effect:

1. Assignment ordering: Auto-generated channel assignments (derived from the participant list) were appended after user assignments. Since HashMap::insert overwrites, the auto values silently replaced user choices. Fix: auto assignments are now inserted first so user assignments win.

2. complete_channels gate: apply_identity_rules and assign_complete_channel_human_id both required the channel to be in complete_channels before applying a channel-wide assignment. RemoteParty is only marked complete for exactly-2-participant meetings, so any manual speaker assignment on RemoteParty in 3+ participant meetings was silently dropped. Fix: remove the complete_channels gate from both functions.

Review & Testing Checklist for Human

• Verify remember_identity asymmetry is safe: remember_identity (speakers.rs ~L159) still gates carry-forward propagation on complete_channels. This is intentionally left to prevent one speaker's identity from overwriting the channel map for multi-speaker channels — but confirm this doesn't cause assignments to "forget" on later segments that rely on carry-forward rather than direct lookup.
• Regression-test the 2-participant case: The common case (self + 1 remote) should still auto-assign labels correctly without user interaction. The ordering change means auto assignments are now processed first instead of last — verify labels still appear correctly.
• Manual end-to-end test: Open a transcript with multiple speakers, click a "Speaker N" label, select a person from the dropdown, and verify the label updates and persists across page navigation.

Notes

• All 62 Rust unit tests pass (60 existing + 2 new covering both bug scenarios).
• Pre-existing TS typecheck failures in editor/session/index.tsx and editor/chat/index.tsx are unrelated (NodeViewComponentProps type issues from react-prosemirror upgrade).

Link to Devin session: https://app.devin.ai/sessions/9d98abf7a843457cafc59d03a88e83e9
Requested by: @ComputelessComputer

[devin-ai-integration]

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