[capi] Environment read-back missing from the C ABI (parity hole vs C++/Python)

[JArmandoAnaya]

What

Environment state can be written through the C ABI (scn_engine_add_environment_action) but never read back: among the scn_engine_* entry points there is no environment getter (only lane-width/state/variable/user-defined-value/date-time/time getters). C++ and Python both expose read-back. docs/user-guide/global-actions.md:370-371 defers this to "the p6-s1 C-API expansion" — p6-s1 (#35) is closed. This is a genuine C-ABI parity hole that the parity audit does not flag (the C++ accessor's absence is presumably recorded as "reasoned", or missed).

Done when

• scn_engine_get_environment_* accessors (weather/time-of-day/road-condition, borrowed-string + status-code conventions per the C ABI rules)
• Python binding + example updated in the same PR (same-PR parity rule)
• scripts/parity_audit.py row flips from reasoned-gap to bound; user-guide note corrected