test: convert assertions to inline snapshots across the codebase

[MatthewMckee4]

Summary

Converts hand-written assert_eq! checks across karva_logging, karva_diagnostic, and karva_metadata to inline insta snapshots. In karva_logging/time.rs the format_duration / format_duration_bracketed tests now pin the exact formatted string. In karva_diagnostic/traceback.rs the filter_traceback, parse_traceback_line, get_traceback_location, and calculate_line_range tests snapshot the full parsed Option<TracebackLocation> / Option<TextRange> shapes instead of building them by hand. In karva_metadata/options.rs the TOML-parsing and Combine merge tests now snapshot the whole struct so every field is pinned, not just the three or four that were previously checked.

Assertions were left as-is where a snapshot would not help: single-integer or single-enum-variant comparisons (e.g. MaxFail and RunHash tests), matches!-style property checks on TestPath variants, is_ok / is_err / len / is_empty checks in the cache and diagnostic stats tests, and roundtrip tests where the entire point is equality. The base branch is test/consolidated-audit (PR #670) so the diff reviews against just this sweep.

Test Plan

• just test (888 tests pass)
• uvx prek run -a (all hooks pass)

[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 1 untouched benchmark

---

_{Comparing test/snapshot-sweep (92e25c2) with test/consolidated-audit (c312ef1)}

[MatthewMckee4]

Folded into #670 via cherry-pick of the karva_diagnostic traceback snapshot commit (27cb339). The karva_logging and karva_metadata conversions from this branch were already made directly on #670 in parallel.