Allow multi-session RTH datasets with session-aware gaps

[sybapp]

Parent

• PRD: ES TradingView-to-Nautilus strategy evolution system #1

What to build

Update the Versioned Dataset Contract and collector behavior so ES RTH datasets can span multiple trading sessions. The completed slice should accept normal overnight or weekend gaps between RTH sessions while still validating that bars are continuous within each session, and it should prevent the collector from reducing an export to only the newest contiguous run.

Acceptance criteria

• Dataset validation allows gaps between distinct RTH sessions while preserving strict 5-minute continuity inside each RTH session.
• Dataset validation reports missing or malformed bars inside a session as contract errors.
• The collector export path can preserve multiple RTH sessions instead of keeping only the newest contiguous run.
• Flat-before-close and session metadata can be derived per session, not from the first bar in the dataset.
• Fixture coverage includes at least two RTH sessions separated by an overnight gap.
• Tests cover both valid multi-session data and invalid intra-session gaps.

Blocked by

None - can start immediately

[sybapp]

Implemented on branch codex/real-nautilus-issues-9-15 in commit 5364156. Verification: npx vitest run tests/datasetContract.test.ts tests/collector.test.ts passed.