Expand optional niche layout beyond raw pointers

[itsfuad]

Goal

Grow optional niche layout while keeping tagged fallback.

Scope

• Keep one niche checker at layout/backend boundary.
• Add non-pointer cases only when invalid representation is compiler-known and ABI-safe.
• Consider unsigned, floats, structs, and arrays.
• Keep source ?T semantics independent from layout choice.

Validation

• Backend layout tests.
• Optional value lowering tests for niche and fallback.