relayburn-reader: port parseClaudeSessionIncremental to Rust (#255)

[willwashburn]

Closes-progress-on #255.

Continues the Rust port of packages/reader/src/claude.ts. PR #262 landed the synchronous core (parse_claude_session + reconcile_claude_session_relationships). This PR fills in the byte-offset resumable variant called out in the issue's acceptance gate ("claude.rs exports the same surface as parseClaudeSession / parseClaudeSessionIncremental / reconcileClaudeSessionRelationships").

Summary

• parse_claude_session_incremental / parse_claude_session_incremental_with_counter — public entry points mirroring parseClaudeSessionIncremental in TS. Read from start_offset, compute end_offset as the byte position of the earliest in-progress assistant message (or the cursor past the last complete newline), and filter every offset-tagged buffer so a resumed call doesn't double-emit.
• ParseIncrementalOptions / ParseIncrementalResult — the option / result types. Carry start_offset and last_user_text in; surface end_offset and last_user_text out so callers can drive multi-pass ingest.
• prescan_nodes — port of TS prescanNodes. When start_offset > 0, pre-reads [0, start_offset) and seeds nodes_by_uuid, evidence, tool-result counters, the next event index, and the last assistant messageId so sidechain turns discovered in the resumed region still resolve their invocation tree, system notifications get the right call_index / event_index, and userTurns get their preceding_message_id populated even when the prior assistant was already ingested.
• record_root_incremental + collect_explicit_claude_relationships_incremental — small offset-tagged variants of the sync-flow root + explicit-fork/continuation collectors so pending_relationships can be filtered by end_offset at emit time.
• Re-exports the new surface from crates/relayburn-reader/src/lib.rs.

The synchronous parser, reconciler, and all existing tests are untouched.

Conformance

11 new lib tests against the shared tests/fixtures/claude/ JSONL fixtures — one per it() case in the describe('parseClaudeSessionIncremental', ...) block of packages/reader/src/claude.test.ts:478-985:

• incremental_reads_whole_file_from_start
• incremental_returns_zero_turns_when_start_at_eof
• incremental_appended_turn_emitted_on_resume
• incremental_defers_in_progress_trailing_message
• incremental_defers_content_for_in_progress_then_emits_after_completion
• incremental_defers_assistant_content_after_in_progress_message
• incremental_skips_incomplete_turn_then_emits_when_completion_arrives
• incremental_preserves_user_prompt_across_resume
• incremental_user_turns_emitted_once_across_resumed_passes
• incremental_seeds_tool_result_event_counters_from_prescan
• incremental_resolves_subagent_tree_via_prescan

```
$ cargo test -p relayburn-reader --lib
test result: ok. 124 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
```

(113 → 124 — no existing test broken.)

Not in this PR (still tracked under #255)

• The non-incremental parity sweep across the rest of claude.test.ts — fork/continuation reconciliation cases, user-turn block sizes, content capture variants. The fixtures already exist; the gap is Rust assertion code, not parser behavior. Landing iteratively keeps each PR reviewable.

Test plan

• cargo build --workspace --all-targets
• cargo test -p relayburn-reader --lib (124 pass)
• cargo test --workspace
• Existing 113 reader tests still pass

🤖 Generated with Claude Code