Add per-test hard timeout configuration

[MatthewMckee4]

Summary

Adds a timeout setting under [test] and a matching --timeout=SECONDS CLI flag that applies a hard per-test limit to every test in the run. Tests exceeding the limit are killed and reported as failures, reusing the same machinery as @karva.tags.timeout (sync tests run inside a ThreadPoolExecutor; async tests are wrapped in asyncio.wait_for). A test-level decorator still wins when both are set, so individual tests can opt into a longer or shorter window.

[tool.karva.profile.default.test]
timeout = 120.0

karva test --timeout=120

Closes #557. The issue also sketched per-filter overrides via [[profile.default.overrides]]; that is left for a follow-up since karva does not yet have an overrides mechanism.

Test Plan

ci

[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 matt/timeout-config (d97f3df) with main (56b23da)}