feat: add deterministic session review projection core

[harshitethic]

Summary

Adds a dependency-free review projection layer for #242 so the local review UI can consume one deterministic representation of ordered evidence instead of re-implementing relationship and filter logic in the frontend.

What it provides

• stable timestamp ordering with recorded source order as a tie breaker
• explicit parent relationship state: none, linked, or orphaned
• reviewer-oriented filters for files, tools, commands, tests, failures, retries, recovery, decisions, privacy transformations, and gaps
• source event IDs, raw/provider-specific data, timestamps, parent IDs, and redaction state preserved for inspection
• missing parents surfaced as gaps without inventing synthetic relationships
• versioned dictionary serialization for future UI/API consumers

Trust boundary

The projector intentionally classifies only recorded facts. Semantic categories such as tests, commands, retries, and recovery require explicit event metadata; it does not guess them from command text or natural-language output. A clean projection therefore is not a claim of complete provider capture.

Tests

tests/test_review.py covers ordering, linked tool results, orphaned results, category filters, retry/recovery metadata, redaction/privacy visibility, and stable timestamp ties.

Documentation

docs/session-review-projection.md documents ordering, relationship semantics, filter rules, raw evidence preservation, and the no-inference boundary.

Scope

This is a focused core for #242, not a claim to close the UI issue. It does not yet replace the dashboard, render annotations, calculate evidence health, or add keyboard/URL navigation. Those layers can build on this projection without duplicating evidence semantics.

Validation

The implementation and tests are stdlib-only beyond AgentTrace's existing TraceEvent model. The connected development machine is currently offline, so I have not claimed local execution; upstream CI is the verification gate for this branch.

Refs #242.