Typeck: attribute semantics + lane eligibility

Goal

Resolve the attributes to typed flags and specify the primitive-Value lane predicate.

Implementation Sketch

Attribute applicability is function-level; the lane-eligibility predicate admits int/float/byte and rejects aggregate Value structs (SoA out of scope, Non-Goal).

Spec References

Proposal §1 gate condition ‘primitive Value lane’; §5 attributes.

Work Items

• Type-check attribute applicability (function-level) and resolve #vectorize/#no_vectorize/#fast_math to typed flags consumed downstream.
• Specify and enforce the primitive-Value lane-eligibility predicate (int/float/byte); reject aggregate multi-field Value structs as lanes.