Add read-only native terminal diagnostic snapshot module

[h0x91b]

Hi, this is Claude (the AI assistant working on this branch) 👋

Summary

Adds a standalone, read-only diagnostics module at src/bun/native-terminal-diagnostics/ — groundwork for HOST-009 on the tmux-removal roadmap (seq 1258). It has no product caller yet; tmux remains the production default and existing behavior is unchanged.

• snapshot.ts — buildDiagnosticsSnapshot(input) shapes injected plain data (mapped from the registry's existing public read APIs: record + live StatusReply + ownership verdict, parser-queue getters, parser-state snapshot) into a versioned, JSON-safe snapshot. Every fact is either {known:true,value} or an explicit {known:false,reason} — unavailable facts are never invented.
• format.ts — a compact, deterministic human-readable formatter over the same snapshot.
• index.ts — barrel re-export.

Design decisions

• Structural redaction (allowlist). The builder only reads the required facts (session/view ids, protocol/runtime/artifact versions, host/shell PIDs, lifecycle, writer presence, timestamps, bounded counters). It never touches the endpoint, bearer token, host executable, start signatures, shell command line, environment, or any parsed terminal output — so nothing in an ignored field can leak. Proven by tests.
• Bounded counters. Non-finite/negative counters degrade to unknown rather than emitting Infinity/NaN, keeping the snapshot JSON-safe.
• Standalone by contract. The module imports nothing from native-terminal-registry (its import-graph isolation test forbids outside references); it declares its own input types, deliberately structurally compatible with the registry's public shapes so a future caller can pass them straight through.
• Injected clock. now/lastAttachAt are injected (no Date.now), so snapshots are reproducible and staleness math is pure.

Follow-up (recorded, intentionally not done here)

lastAttachAt is not available from any current public read API (the record has created/updated, StatusReply has the shell startedAt). The module accepts it as an optional input and reports unknown otherwise; populating it would require the registry/host to track an attach timestamp — out of scope for this isolated module.

Includes a changelog entry and decision record 164. 14 focused pure tests (healthy / stale / partial / missing / redacted, boundedness, deterministic ordering); full suite green.