[Rust port] relayburn-analyze: overhead attribution (1e-9 USD precision gate)

[willwashburn]

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

Context

Overhead attribution is the surface added in #236: it answers "how much of this session's spend went to system overhead (CLAUDE.md / AGENTS.md context loaded into every prompt) vs actual user-asked work?" Small in source LoC (~122) but it's the fresh divergence-risk surface called out in #244's acceptance: per-file, per-section USD totals must match TS within 1e-9 USD.

This is a leaf module — it doesn't compose other detectors — so it can land independently of the other sub-issues, in parallel with the detector PRs.

Scope

overhead.rs (port of overhead.ts, ~122 LoC)

• Port OverheadFileKind (claude-md | agents-md), OverheadHarness (claude-code | codex | opencode).
• Port the full attribution type tree:
  • OverheadFile, ParsedOverheadFile, OverheadFileAttribution, OverheadAttribution, AttributeOverheadInput (already enumerated in [Rust port] relayburn-analyze: pricing, hotspots, overhead #244's scope).
  • OverheadResult, OverheadSection, OverheadFileSummary, OverheadPerFileEntry, OverheadAttributionDetail, OverheadSectionCost, OverheadTrimResult, OverheadTrimRecommendation.
• Port findOverheadFiles(projectPath) — discovers CLAUDE.md and AGENTS.md files across the project tree, including parent directories and .claude/ / .codex/ subdirs.
• Port loadOverheadFile(file) — reads + parses a single overhead file via the shared markdown parser (reuse parseClaudeMd from [Rust port] relayburn-analyze: subagent-tree + claude-md attribution #272's claude_md.rs).
• Port attributeOverhead(input) — the 1e-9-precision-sensitive core. Computes per-section USD attribution by weighting input-token cost by section token share.
• Port describeAppliesTo(applies_to) — returns the human-readable label ("Claude Code only", "Codex + OpenCode", etc.).
• Port overheadTrim() if exported (check index.ts re-exports — surface lives in this module per the parent issue scope).

Precision contract

The 1e-9 USD gate is the headline acceptance criterion of #244. Concretely:

• Use f64 arithmetic, same operation order as TS (token-share × per-token rate, summed in section-iteration order).
• Section iteration must be deterministic: order by (file_path, section_index) ascending — match TS sort key exactly.
• Avoid intermediate rounding. Round only at the serialization boundary if the consumer requests a fixed precision; the in-memory f64 carries the full double precision.
• A regression test computes overhead for a multi-file fixture corpus and asserts (rust_total_usd - ts_total_usd).abs() < 1e-9 per (file, section) pair.

Conformance gate

• packages/analyze/src/overhead.test.ts (230 lines).

Acceptance

• cargo test -p relayburn-analyze green for overhead.
• Public surface: attributeOverhead, describeAppliesTo, findOverheadFiles, loadOverheadFile, plus the full attribution type tree listed above.
• 1e-9 USD precision gate verified against the TS fixture corpus (per-file, per-section).
• findOverheadFiles traversal order deterministic and matches TS (BFS by directory depth, alphabetical within depth).

Files

• packages/analyze/src/overhead.ts + overhead.test.ts