test(axis): expand blackbox unit test coverage

[EffortlessSteven]

Summary

Expands unit-test coverage of crates/flight-axis/src/blackbox.rs. Before this PR the 705-LOC blackbox/annotation module had only 7 inline tests.

Adds 19 new #[test] functions (7 → 26 in blackbox::tests) covering:

• Default impl
• set_enabled(false) buffer clearing
• clear_buffer no-flush semantics
• flush() empty-buffer no-op
• Each previously uncovered event variant: ConflictCleared, PreFaultCapture, CapabilityModeChanged, OutputClamped
• All three CapabilityMode variants (Full / Demo / Kid)
• Resolution failure path
• Disabled-mode suppression for every annotate_* method
• set_max_buffer_size flush thresholds (at / under / exact boundary)
• From<&ConflictMetadata> conversion
• ResolutionDetails::new / with_metrics semantics including round-trip clearing

All tests run on the non-RT harness thread. Allocations are explicit and ADR-004 compliant (test code is not RT). No production code modified.

Test plan

• cargo test -p flight-axis — 745 unit + 25 doctests + 7 integration suites, 0 failures
• cargo fmt -- --check and cargo clippy clean on blackbox.rs

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: 36a243d2-4730-446d-83b0-c20a2ee5c897

📥 Commits

Reviewing files that changed from the base of the PR and between 7d026c3 and d55de57.

📒 Files selected for processing (1)

• crates/flight-axis/src/blackbox.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch claude/coverage/axis-blackbox-tests

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