Phase 0e - Observability substrate (DONE)

The plumbing for D046 and D047, before there is much to report - so that every later phase adds to it naturally instead of retrofitting.

• Run identity. One id shared by the log, the trace, and the report.
• Developer log to hardware plus a rolling file, structured fields rather than formatted prose, with the level contract from D047 documented and honoured.
• Run report: versioned schema, emitter, and the diff-against-previous-run machinery. Thin at first - it has little to describe until phase 1 - but the shape is what later phases fill in.
• Retention: 72-hour purge on startup, a manual purge command, and a size budget.

tracing returns to each core crate here as that crate gains real log calls, not speculatively - it was pruned during the dependency cleanup because it was genuinely unused.

Observable result: every run produces a correlated log, an optional trace, and a bounded machine-readable report that says what it was given and what it did.