test(bf-3m477): capture timing flag outputs

Test each cargo nextest timing flag from bf-4wz6v documentation:
- --message-format=libtest-json-plus (2.0M, JSON with exec_time)
- --message-format=libtest-json (2.0M, JSON format)
- --message-format-version=0.1 (2.0M, versioned JSON)
- --message-format-version=1.0 (error, invalid version range)
- --message-format-version=1 (error, invalid format)
- --status-level=slow (112K, slow test detection)
- --final-status-level=slow (412K, slow test summary)

Acceptance criteria:
- PASS: Output file exists for each tested flag (8 files)
- PASS: Each file named after the flag it tests
- PASS: At least 3 flags tested (7 unique flags)

Closes bf-3m477

notes/bf-3m477.md

@@ -0,0 +1,72 @@
+# bf-3m477: Test Each Timing Flag and Capture Output
+
+## Task
+Test each timing flag and capture output for analysis.
+
+## Implementation
+
+### Flags Tested
+
+All 5 timing flags from bf-4wz6v documentation were tested:
+
+1. **`--message-format=libtest-json-plus`** ✅
+   - Command: `NEXTEST_EXPERIMENTAL_LIBTEST_JSON=1 cargo nextest run -p pdftract-core --lib --message-format=libtest-json-plus`
+   - Output: `notes/bf-mge0o-message-format-libtest-json-plus.log` (2.0M)
+   - Result: SUCCESS - JSON output with `exec_time` fields per test
+
+2. **`--message-format-version=0.1`** ✅
+   - Command: `NEXTEST_EXPERIMENTAL_LIBTEST_JSON=1 cargo nextest run -p pdftract-core --lib --message-format=libtest-json-plus --message-format-version=0.1`
+   - Output: `notes/bf-mge0o-message-format-version-0.1.log` (2.0M)
+   - Result: SUCCESS - JSON output with versioned schema (version must be in `major.minor` format, major must be 0)
+
+3. **`--status-level=slow`** ✅
+   - Command: `cargo nextest run -p pdftract-core --lib --status-level=slow`
+   - Output: `notes/bf-mge0o-status-level-slow.log` (112K)
+   - Result: SUCCESS - Standard output format, marks slow tests during execution
+
+4. **`--final-status-level=slow`** ✅
+   - Command: `cargo nextest run -p pdftract-core --lib --final-status-level=slow`
+   - Output: `notes/bf-mge0o-final-status-level-slow.log` (412K)
+   - Result: SUCCESS - Standard output format with slow test summary at end
+
+5. **`--message-format=libtest-json`** ✅
+   - Command: `NEXTEST_EXPERIMENTAL_LIBTEST_JSON=1 cargo nextest run -p pdftract-core --lib --message-format=libtest-json`
+   - Output: `notes/bf-mge0o-message-format-libtest-json.log` (2.0M)
+   - Result: SUCCESS - JSON output without nextest-specific extensions
+
+### Key Findings
+
+1. **Experimental flag requirement**: Both `libtest-json` and `libtest-json-plus` require `NEXTEST_EXPERIMENTAL_LIBTEST_JSON=1` environment variable
+2. **Version format**: `--message-format-version` requires `major.minor` format (e.g., `0.1`), not just `1`
+3. **Version constraints**: Major version must be 0 (range 0..1)
+4. **JSON format**: Both JSON formats produce per-test `exec_time` fields for timing data
+5. **Slow test flags**: `--status-level=slow` and `--final-status-level=slow` work without experimental flags
+
+### Test Suite
+- 2,902 lib tests from `pdftract-core`
+- Runtime: ~4.8s per run
+- 2857 passed, 45 failed, 2 skipped
+
+## Acceptance Criteria
+
+- ✅ **PASS**: Output file exists for each tested flag (5 files created)
+- ✅ **PASS**: Each file is named after the flag it tests
+- ✅ **PASS**: At least 3 flags are tested (5 tested, exceeds requirement)
+
+## Output Files Created
+
+1. `notes/bf-mge0o-message-format-libtest-json-plus.log` (2.0M)
+2. `notes/bf-mge0o-message-format-version-0.1.log` (2.0M)
+3. `notes/bf-mge0o-status-level-slow.log` (112K)
+4. `notes/bf-mge0o-final-status-level-slow.log` (412K)
+5. `notes/bf-mge0o-message-format-libtest-json.log` (2.0M)
+
+## References
+
+- Third child of split bf-mge0o
+- Depends on bf-2n36a (baseline comparison)
+- Documentation from bf-4wz6v timing flag research
+
+## Verification
+
+All timing flags successfully executed and output captured. The JSON formats (libtest-json and libtest-json-plus) both provide per-test timing data via the `exec_time` field, making them suitable for programmatic consumption of test execution times.

notes/bf-mge0o-message-format-version-1.0.log

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+error: error parsing message format version
+
+Caused by:
+  invalid format version: 1.0
+
+Caused by:
+  version component `major` value 1 is out of range 0..1

notes/bf-mge0o-message-format-version-1.log

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+error: error parsing message format version
+
+Caused by:
+  invalid format version: 1
+
+Caused by:
+  expected format version in form of `<major>.<minor>`