fix: correct EventBus capacity accounting

[MagMueller]

Summary

• count queued and processing events as non-overlapping capacity inputs
• report queue saturation with accurate queue and processing counts
• replace the misleading capacity regression with a focused 50-event queue test

Tests

• uv run pytest -q tests/
• uv run ruff check .
• uv run ruff format --check .
• uv run pyright

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Summary by cubic

Fixes EventBus capacity checks by separating queued from processing events and enforcing a queue‑full condition to prevent premature capacity errors.

• Bug Fixes
  • Compute pending as queue size + started events only; stop double counting queued items in history.
  • Raise capacity errors with a clear reason ('100 max' or 'queue full') and report separate Queue and Processing counts.
  • Replace the 100-limit stress test with a focused 50-event queue saturation test that validates accurate counts and messaging.

^{Written for commit 3ab4c41. Summary will update on new commits.}

[claude]

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