[Rust port] relayburn-analyze: subagent-tree + claude-md attribution

[willwashburn]

Parent: #244
Depends on: #242, #266 (foundation), #268 (findings)

Context

Two attribution modules that walk session structure and produce per-node cost rollups. They land together because both produce tree-shaped output and share the "build a hierarchy then attribute cost to each node" pattern.

Scope

subagent_tree.rs (port of subagent-tree.ts, ~640 LoC)

• Port SubagentTreeNode, BuildSubagentTreeOptions, SubagentTypeStats types.
• Port buildSubagentTree(turns, options) — walks parent-uuid chains to construct the tree of dispatched subagents per session.
• Port aggregateSubagentTypeStats(turns) — flat aggregation over subagent type names.
• Sidechain reconciliation rules (sidechain → parent linkage) are upstream in relayburn-reader; this module just consumes the subagentType / parentUuid fields.

claude_md.rs (port of claude-md.ts, ~402 LoC)

• Port MarkdownSection, ParsedClaudeMd, SessionClaudeMdCost, SectionCost, ClaudeMdAttributionResult, AttributeClaudeMdInput, TrimRecommendation types.
• Port findClaudeMdFiles(projectPath) — filesystem walk for CLAUDE.md in cwd + parents + .claude/CLAUDE.md.
• Port loadClaudeMdFile(path), parseClaudeMd(path, text) — markdown section parser; lines starting with # open new sections, body accumulates until next header, indentation level encodes section nesting.
• Port attributeClaudeMd(input) — attributes per-session token cost to specific markdown sections weighted by token share.
• Port buildTrimRecommendations(result) and renderUnifiedDiffForRecommendation(rec, parsed) — produces a unified-diff string for a section to be removed; keep the diff format (---/+++/@@ headers, - line prefixes) byte-identical to TS, since CLI consumers may grep on it.

Conformance gate

• packages/analyze/src/subagent-tree.test.ts (326 lines)
• packages/analyze/src/claude-md.test.ts (360 lines)

Acceptance

• cargo test -p relayburn-analyze green for both modules.
• Public surface for subagent: buildSubagentTree, aggregateSubagentTypeStats, BuildSubagentTreeOptions, SubagentTreeNode, SubagentTypeStats.
• Public surface for claude-md: findClaudeMdFiles, loadClaudeMdFile, parseClaudeMd, attributeClaudeMd, buildTrimRecommendations, renderUnifiedDiffForRecommendation, plus all listed types.
• Tree node ordering deterministic (sort by ts asc, then uuid asc within siblings) and matches TS.
• Unified-diff output matches TS byte-for-byte on the trim-recommendation fixture corpus.

Files

• packages/analyze/src/subagent-tree.ts + subagent-tree.test.ts
• packages/analyze/src/claude-md.ts + claude-md.test.ts