Add point-in-time index membership timelines

[Pigbibi]

Summary

• add point-in-time index membership timelines for CSI 300, CSI 1000, and CSI 500
• add versioned manifests with explicit coverage and comparison semantics
• fail closed before PIT coverage is proven; incomplete historical membership is not emitted as comparable change data
• add duplicate/reverse-capture rejection and atomic rollback/recovery coverage

Scope

• PIT membership timeline queries use effective-date intervals rather than a current constituent list
• pre-coverage queries fail closed instead of silently treating the first observed snapshot as complete history
• duplicate captures and reverse/invalid effective-date ranges are rejected
• CSV and manifest publication is atomic with recovery preserving the last valid pair
• existing CSV bytes are unchanged; this PR only adds manifests plus implementation/tests

Validation

• focused tests: 35 passed
• runnable test collection excluding the known missing sibling strategy module: 97 passed
• full collection reproduces the existing missing import for cn_chinext_growth_momentum_quality_snapshot; the related implementation and test paths are unchanged by this PR
• Ruff, compileall, git diff --check, and sensitive-field diff scan passed

Known baseline blocker

The full collection cannot import cn_equity_strategies.strategies.cn_chinext_growth_momentum_quality_snapshot. This is a pre-existing sibling-repository dependency/fixture gap, not changed here and not fixed in this PR.

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