Improvements based on rigorous maintainer review and code verification:

P0 - CRITICAL (Missing Coverage):
- Add finish_reason propagation validation in test_request_finish_metrics_completeness
  * Verify FinishReason.LENGTH → "length" string conversion (via __str__)
  * Assert finish_reason appears in CompletionOutput.finish_reason field
  * Assert RequestOutput.finished reflects completion state
  * Verified: FinishReason.LENGTH stringifies to "length" via FINISH_REASON_STRINGS[1]

P1 - HIGH (Strengthen Weak Assertions):
- Replace `> 0.0` checks with exact equality for first_token_ts and last_token_ts
  * first_token_ts == 1100.0 (set on first update_from_output while is_prefilling=True)
  * last_token_ts == 1170.0 (set from engine_core_timestamp in final step)
  * Verified: Both are direct assignments from engine_core_timestamp (no transformation)
- Add timing invariants: queued_ts <= scheduled_ts, first_token_ts <= last_token_ts
  * Removed scheduled_ts <= first_token_ts (not guaranteed in this test scenario)

P2 - MEDIUM (Documentation):
- Enhanced test_journey_events_populate_prometheus_timestamps docstring
  * Document first-write-wins conditions: source > 0.0 AND dest == 0.0
  * Reference exact code locations (output_processor.py:535,537)
  * Add comment explaining copy condition sensitivity
- Enhanced test_timestamp_propagation_batch_processing docstring
  * Define "isolation": each request's stats contain only its own timestamps
  * Clarify no cross-contamination between requests in batch
- Add clock domain documentation to test_request_finish_metrics_completeness
  * arrival_time: wall-clock (time.time()) - verified from input_processor.py:477
  * queued_ts/scheduled_ts/first/last_token_ts: monotonic - verified from scheduler.py
  * Warning: DO NOT compare across domains in production code

P3 - LOW (Internal State Check):
- Label request_states cleanup check as "leak-guard invariant"
  * Document guarantee: output_processor.py:587 unconditional pop on finish
  * Valid contract verification, not just implementation detail

All changes verified against actual code behavior. No assumptions or speculation.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

CRITICAL CLARIFICATION based on code verification:

The field name "first_token_ts" is semantically misleading. It does NOT mean
"timestamp when first token is generated" but rather "timestamp of first
prefill update" (even if no tokens generated).

Code Evidence (stats.py:288-289):
```python
if is_prefilling:
    req_stats.first_token_ts = engine_core_timestamp
```

Condition: ONLY checks `is_prefilling` flag (no check on token emission)

Order of Operations (output_processor.py:541-552):
1. _update_stats_from_output() called (sets first_token_ts)
2. THEN is_prefilling = False

Test Execution:
- Step 1 (new_token_ids=[], timestamp=1100.0): first_token_ts = 1100.0 ✓
- Step 2 (new_token_ids=[42], timestamp=1150.0): first_token_ts unchanged (is_prefilling=False)

Assertion is CORRECT (1100.0), but semantic meaning needed clarification.

Changes:
- Enhanced inline comment explaining condition (stats.py:288 reference)
- Note semantic confusion: "first_token_ts" ≠ "first token generated"
- Clarify it means "first prefill update timestamp"
- Add docstring section explaining this behavior
- Reference exact test steps to avoid future confusion

No behavior changes. Documentation only.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Revised wording to be neutral and professional:
- "Semantic confusion" → "Note on first_token_ts definition"
- "doesn't mean" → "Currently defined as"
- "Despite the name" → "Currently defined as"
- "Does NOT check" → "Condition checks ... only"
- "NOT 1150.0" → "not step 2"

Preserves all technical accuracy while avoiding judgmental language.

No behavioral changes. Documentation wording only.