feat: preserve keyboard shortcuts as typed capture events

[abrichr]

Outcome

Capture now preserves demonstrated modifier chords as typed key.shortcut events rather than flattening them into text. This gives Flow a stable, provenance-bearing input for governed hotkey actuation while retaining ordinary Shift+character typing.

Included

• canonical Ctrl/Alt/Shift/Meta chord representation
• retained raw keyboard children and structural evidence
• high-level Action.keys exposure
• SQLite event registration across both storage implementations
• focused model, processing, and persistence coverage

Verification

• 342 passed, 8 skipped using the CI-equivalent suite (browser-bridge file excluded by repository CI)
• focused 72 passed
• Ruff checks passed
• wheel and sdist built and scripts/verify_distribution.py passed

This is the Capture half of the cross-repository action-semantics work. Flow will consume shortcuts alongside existing captured right-click and drag evidence in a follow-up once the active remote-runtime branches merge.