`re corpus` — cross-capture corpus analysis

[hexsecs]

Summary

Extend reverse-engineering helpers to operate on a corpus of captures so
analysts can surface fleet-level patterns (PGN drift, signal-cycle
anomalies, ID coverage changes across vehicles).

Acceptance Criteria

• New command canarchy re corpus <file...> (or --corpus-glob) ingests multiple captures and emits an aggregate JSON envelope.
• Output includes per-ID coverage matrix across captures, cycle-time drift, DM1 fault diffs (J1939), and signal-stability indicators.
• Builds on existing primitives in src/canarchy/re_processors.py; no new heavy dependencies.
• Tests cover at least three small fixtures and assert aggregate metrics.
• MCP tool mirror is added with the same bounded semantics used elsewhere (no streaming).
• docs/feature-matrix.md Reverse-engineering depth row is updated.

Notes

Part of #295.