recipe: validate constraint measurement paths at load time

[mchmarny]

Follow-up to #1542 (design section 10). pkg/constraints.Evaluate distinguishes malformed constraint paths (ErrCodeInvalidRequest, now fails resolution closed) from values absent in the snapshot (ErrCodeNotFound, graceful exclusion by design). A grammatically valid but typo'd constraint path evaluates as NotFound and still degrades silently for unstated dimensions. Validate constraint paths against the measurement catalog when loading recipe data so typos fail at load time instead of masquerading as missing measurements.

[mchmarny]

Triaged: added to the v1 milestone (API Stability).

Moving typo'd constraint paths from silent NotFound degradation to a load-time failure is a fail-closed semantics change on a frozen surface (ROADMAP §2). Pre-v1 work.

[mchmarny]

Triaged: P2 → P1 (Ready).

This is the cleanest false-PASS on the board. A grammatically valid but typo'd constraint path evaluates as ErrCodeNotFound, which the resolver treats as "value absent from the snapshot — exclude gracefully". The recipe then resolves successfully with that constraint silently never evaluated — a recipe passing because it skipped a gate.

It is also an input check for #2116 (aicr recipe validate): shipping the contributor-facing validator without it means shipping a validator that misses the highest-value class of authoring typo.