docs: call the header 'declared', not 'shipped'

Round-6 nit (Kimi, non-blocking): C_CODING_STANDARDS.md §4 called
include/sensorwatch/sensorwatch.h 'shipped', but the project is Python-only and
the header isn't part of any release yet -- 'declared' matches the declaration-only
reality. One-word wording fix; no code change.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>