Reject non-finite flux-summary numerics

[FlorianPfaff]

Summary

• validate summarize_flux() timing metadata before computing scientific summaries
• reject fractional/non-finite frame counts and non-finite frame rate, duration, or belt-velocity metadata
• reject non-finite derived summary values, including arithmetic overflow in velocity conversion
• make beltmap-flux-summary reject NaN/infinite frame rates during argument parsing, before writing artifacts
• add regressions for package autoloading, invalid metadata, derived overflow, CLI failure behavior, and patch reload idempotence

Root cause

beltmap-flux-summary parsed --frame-rate-hz with plain float(). Comparisons such as nan <= 0 are false, so NaN was forwarded to summarize_flux() instead of being treated as absent or invalid.

The shared summarizer also accepted non-finite timing metadata directly. It stores frame_rate_hz in the result and multiplies per-frame velocities by it, so NaN, infinity, or finite arithmetic overflow could reach particle_flux_summary.json as bare NaN/Infinity tokens. Python's encoder permits those by default, but they are not valid strict JSON and can also make the reported science metrics meaningless.

Impact

Malformed timing metadata now fails clearly rather than producing non-standard JSON or non-finite velocity/flux statistics. Normal finite inputs and the CLI's existing zero-as-unspecified behavior are unchanged.

Validation

• isolated executable harness reproduced and rejected NaN frame rates
• isolated executable harness rejected overflowed derived velocity values
• repeated patch reloads preserved the true original summarizer
• all changed Python sources syntax-parse successfully
• branch is 4 commits ahead of and 0 commits behind main

GitHub Actions will provide the repository-wide test and lint result.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ ACTION	actionlint	10		0	0	0.93s
✅ COPYPASTE	jscpd	yes		no	no	3.47s
✅ MARKDOWN	markdownlint	24	0	0	0	2.25s
✅ PYTHON	ruff	208	1	0	0	0.36s
✅ REPOSITORY	git_diff	yes		no	no	0.07s
✅ YAML	prettier	12	0	0	0	1.45s
✅ YAML	v8r	12		0	0	5.02s
✅ YAML	yamllint	12		0	0	0.88s

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