Bound watched field change queues

[Mirochill]

Summary

• Bound sync and async watched-field change queues with a default capacity of 1024.
• Drop the oldest buffered change when a slow consumer lets the queue fill, and expose dropped_changes on watched fields.
• Add regression coverage for sync and async queue overflow behavior and max_queue_size wiring.

Related issues

Closes #100

Test plan

• git diff --check
• git diff --cached --check
• git show --check HEAD
• make lint passes
• make typecheck passes
• make test passes

Not run locally: make lint, make typecheck, make test.

Checklist

• SDK regenerated if proto changed in decree (not applicable)
• Breaking change flagged in title/description if applicable (not applicable)
• Changelog/release note added if applicable (not applicable)

[github-actions]

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