PAR-008: NEC-5 validation-manual coverage matrix (Phase 2)

[dc0sk]

Resolution criteria: NEC-5 Validation Manual scenario classes mapped to fnec-rust in-scope equivalents; each mapped class has at least one reproducible corpus test with explicit tolerance gating; known out-of-scope classes are documented with rationale.

[dc0sk]

Implemented in #24. Added the NEC-5 validation coverage matrix, including mapping of scenario classes, explicit interpretation rules for gating, and rationale for out-of-scope classes.