Migrate to chiptime

Line-by-line API mappings for moving existing code to chiptime. The guides focus on mechanics: how each concept maps, what you can delete after switching, and where behavior differs so nothing surprises you.

• Migrate from fitparse
• Migrate from fitdecode

Two behavior notes that apply to any migration:

1. Damaged files parse instead of raising (in the default lenient mode). Code structured around exception handling moves to checking result.ok, result.errors, and provenance[] — and strict mode restores raise-on-first-violation semantics where you want them.
2. None is trustworthy. FIT sentinel values are converted to None during decode, and zero is preserved as a real measurement, so hand-rolled sentinel guards can be deleted.