[Vmnebula]

Fixes #3578.

A NaN jump permanently corrupts MockClock's virtual time, so subsequent valid jumps cannot recover it. Reject NaN before modifying the clock, while retaining the existing negative-jump error and zero/positive-jump behavior.

Adds regression coverage for stationary and advancing clocks, verifies that rejection preserves the time and permits later valid jumps, and updates the documented exception and release note.

Verification

• New regressions on unchanged upstream: 2 failed (missing ValueError).
• Mock-clock tests including slow tests: 9 passed.
• Default source test suite: 829 passed, 72 skipped, 2 xfailed.
• mypy --platform linux src/trio: passed, 144 source files.
• Changed-file repository pre-commit checks: passed, including Black, Ruff, spelling and Sphinx lint.

Broader checks and local limitations (Python 3.12.3, Linux)

The full slow suite produced 876 passed, 23 skipped, 2 xfailed and 2 failures. Both failures are existing test_static_tool_sees_class_members cases for Jedi/Mypy reporting trio.WindowsPath.ntpath; reproduced unchanged on a clean upstream worktree at 93e6b2e.

The repository-wide Mypy command including docs encounters an existing docs/source/conf.py:285 Sphinx inventory typing error, also reproduced on that clean upstream worktree. Library/source Mypy passes with the repository's default Python 3.10 target.

AI-assisted implementation and local verification; no separate human review is claimed.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00000%. Comparing base (93e6b2e) to head (136ed99).

Additional details and impacted files

@@               Coverage Diff               @@
##                 main        #3585   +/-   ##
===============================================
  Coverage   100.00000%   100.00000%           
===============================================
  Files             128          128           
  Lines           19555        19571   +16     
  Branches         1330         1331    +1     
===============================================
+ Hits            19555        19571   +16     

Files with missing lines	Coverage Δ
src/trio/_core/_mock_clock.py	100.00000% <100.00000%> (ø)
src/trio/_core/_tests/test_mock_clock.py	100.00000% <100.00000%> (ø)

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[A5rocks]

#3579 (comment)