Add name= parameter to assert_snapshot for named snapshots

[MatthewMckee4]

Summary

• Add name= keyword argument to karva.assert_snapshot() for user-defined snapshot names
• Named snapshots produce files like test__test_page--header.snap using the -- convention
• name= and inline= are mutually exclusive (enforced via typing overloads and runtime check)
• Update base_function_name() to strip --name suffixes so karva snapshot prune works correctly with named snapshots
• Parametrized named snapshots produce test__test_func--name(param=value).snap

Test plan

• Unit tests for compute_named_snapshot (plain and parametrized)
• Unit tests for base_function_name with -- suffixes
• Integration test: named snapshot creates correct file
• Integration test: named snapshot matches on second run
• Integration test: named snapshot mismatch shows correct diff
• Integration test: multiple named snapshots in same test
• Integration test: mixed named + unnamed in same test
• Integration test: name= + inline= raises TypeError
• Integration test: named parametrized snapshots
• Integration test: named snapshot prune
• All 574 tests pass, prek run -a clean

[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

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

✅ 1 untouched benchmark

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_{Comparing add-named-snapshots (22a765c) with main (42f0ff5)}