reader: assign turn membership by parentUuid chain walk (#433)

[willwashburn]

Closes#433.

Summary

Groups Claude turn rows by walking the parentUuid chain up to the nearest user-prompt ancestor, instead of relying on file order / timestamp windows for user-prompt-text → assistant mapping in activity classification.

• New parent_chain submodule with ChainNode trait, group_by_parent_chain, find_turn_root (the API surface the issue specifies; unit-tested).
• Hot path uses a lighter nearest_user_prompt_root helper that returns Option<String> so the caller can cleanly fall back to the file-order map for rows missing both uuid and parentUuid.
• Cycle guard via visited set.
• New user_text_by_uuid map and is_user_prompt field on LineNode to thread the UUID-keyed lookup through classification.

Scope clarification

Burn's reader already collapses multi-block assistants correctly via message_id — turn counts and message_id grouping are unchanged. The actual mis-attribution this fixes is user-prompt-text → assistant mapping for activity classification, where out-of-order JSONL flushes and interrupt-resume gaps would previously cause incorrect attribution.

Codex/opencode readers unchanged (no parentUuid field).

Open question

group_by_parent_chain and find_turn_root are declared #[allow(dead_code)] — they satisfy the issue's specified API surface and are unit-tested, but the hot path uses the lighter single-row helper. Happy to either remove them or wire them through if reviewers prefer; doc comments explain the trade-off.

Test plan

• parent_chain unit tests (10 new): chain walk, root detection, cycle guard, missing-uuid fallback, multi-root file
• End-to-end reader tests (3 new): out-of-order rows, interrupt+resume pattern, classification correctness
• New fixtures: parent-chain-out-of-order.jsonl, parent-chain-interrupt-resume.jsonl
• cargo build --workspace clean
• cargo test --workspace — 813 passed, 0 failed
• BURN_GOLDEN=1 cargo test --test golden — 5/5
• cargo clippy -p relayburn-sdk --lib clean on new code

Out of scope

• Codex/opencode reader changes (no parentUuid field; documented asymmetry).
• No schema change — pure reader-side classification fix.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 8bce6e0b-a406-40b1-b36c-9eabae2106cf

📥 Commits

Reviewing files that changed from the base of the PR and between 33cc446 and 82a35cd.

📒 Files selected for processing (5)

• CHANGELOG.md
• crates/relayburn-sdk/src/reader/claude.rs
• crates/relayburn-sdk/src/reader/claude/parent_chain.rs
• tests/fixtures/claude/parent-chain-interrupt-resume.jsonl
• tests/fixtures/claude/parent-chain-out-of-order.jsonl

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch claude/burn-433-parent-uuid-grouping

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