feat(engine): trusted-ingestion parse path + detection cap

[sepehr-safari]

Summary

Lift Studio past the browser toolkit's ceiling for traces the user opens — the M8
"beyond-browser capacity" differentiator — while keeping the strict caps for
untrusted input.

• Ingestion trust boundary (ADR-0007). The size/event caps are now a
  parser.Limits value chosen by the source. parseTrace keeps the browser-scale
  untrusted_limits (10 MB / 10k) unchanged; parseTraceTrusted applies
  trusted_limits (256 MB / 2M) for files the user explicitly opened. The
  workspace opens files trusted; future live capture (S5) stays untrusted.
• 500k / 100 MB capacity, proven. A 500 000-event JSONL trace parses (one line
  at a time — no whole-file JSON tree), correlates, and the timeline's virtual
  window stays viewport-sized. All on the load path is O(n) / O(n·sessions).
• Detection cap (honest, surfaced). Several detection rules are O(n²) in the
  event count (they mirror the toolkit's algorithms — fine at 10k, a load-time
  stall at 500k). Past detection.max_events_for_detection (50 000) the workspace
  skips detection: the trace still parses, correlates, and is fully
  inspectable, and the status bar shows detection skipped (large trace) instead
  of a misleading empty result. The O(n) rewrite is tracked in perf(engine): O(n) detection rules for dataset-scale traces #36 (with the
  contract-v1 goldens as the bit-exactness safety net).

Verification

• native test -Dplatform=null — 69/69 pass (macOS; CI runs Linux too). New
  coverage: the trusted path parses a 500 000-event trace; parseTraceWithLimits
  honors explicit byte/event caps; the untrusted default still rejects >10 MB /

  10k; a trace past the detection cap loads fully with detection_skipped set and
  zero failures; the sample stays under the cap and runs detection. The 15
  conformance goldens still match (the harness parses untrusted, uncapped).

• native check --strict clean; native build produces the ReleaseFast binary.

Security note

This raises an untrusted-input bound only for trusted sources (files the user
opened), keyed to source, not globally. The untrusted parseTrace path — the one
future live-socket capture will use — is unchanged. The trusted cap is bounded
(256 MB / 2M), so a pathological trusted file still cannot exhaust memory.

Closes #29