perf(secure-messaging): isolate and bound plugin work

[ProtonDev-sys]

Summary

Complete Secure Messaging performance and isolation audit.

Changes

• bounded FIFO work queues for decryption, attachments, unfurls, and key reviews
• deduplicated key-announcement review work
• lower-copy cryptographic and attachment buffers
• indexed message grouping with channel-scoped notifications
• bounded renderer update batches
• conversation-scoped message-length bypass state
• hardened startup, shutdown, reconnect, account-switch, cancellation, and cleanup paths
• permanent performance-boundary regression coverage

Validation

The source branch is based directly on nightly and is not behind it. The complete Test and autofix workflows must pass on this pull request before merge.

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