fix(tests): CI-safe timing threshold for partial-cache latency test

[paddymul]

Summary

• CI's Python / Test (Max Versions) (3.13) job failed on main (run 36148490455): test_huge_dataframe_partial_cache_scenario asserted initial_time < 0.5 and got 0.7584092617034912.
• That assertion measures wall-clock widget-construction latency, which is sensitive to shared CI-runner scheduling variance, not the cache behavior under test. Locally the same code path consistently finishes in well under 0.1s across a dozen runs (traced with the executor's own logging - the only variable-cost step, one-time mp_timeout calibration, is paid by an earlier test in the same file and was already warm by the time this test ran).
• No commit between the last green Checks run and the failing one touched buckaroo/file_cache/ or the widget construction path, so this isn't a regression - it's a hardcoded absolute-time bound that doesn't hold up under runner load.
• Widened the bound to 3.0s, matching the CI_TIMEOUT headroom this same test directory (tests/unit/file_cache/mp_test_utils.py) already uses for the same class of fork/scheduling-bootstrap variance.

Test plan

• uv run pytest tests/unit/file_cache/test_partial_cache_computation.py -vv (all 3 tests pass)
• uv run pytest tests/unit/ -m "not slow" (1171 passed, 5 skipped)
• CI green on this PR

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[github-actions]

📦 TestPyPI package published

pip install --index-strategy unsafe-best-match --index-url https://test.pypi.org/simple/ --extra-index-url https://pypi.org/simple/ buckaroo==0.15.8.dev36458802452

or with uv:

uv pip install --index-strategy unsafe-best-match --index-url https://test.pypi.org/simple/ --extra-index-url https://pypi.org/simple/ buckaroo==0.15.8.dev36458802452

MCP server for Claude Code

claude mcp add buckaroo-table -- uvx --from "buckaroo[mcp]==0.15.8.dev36458802452" --index-strategy unsafe-best-match --index-url https://test.pypi.org/simple/ --extra-index-url https://pypi.org/simple/ buckaroo-table

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