Add optional niche layouts

[itsfuad]

Goal

Introduce optional niche layouts while preserving tagged fallback and unchanged source semantics.

Current state

LLVM currently uses tagged {present, value} layout for all ?T, including ?*T. Some design docs still describe pointer niches as delivered; correct those status claims before adding optimization.

Scope

• Keep one canonical niche decision at type-layout or backend boundary.
• Add a niche only when invalid representation is compiler-known, target-correct, and ABI-safe.
• Evaluate owned and raw pointer-like carriers first; consider integers, floats, structs, and arrays only with proved invalid bit patterns.
• Preserve tagged fallback for every type without valid niche.
• Keep construction, presence checks, payload access, equality, cleanup, and ABI behavior representation-independent.

Relationships

• Add optional narrowing and payload access #28 defines user-facing narrowing and payload semantics.
• This issue may use Add optional narrowing and payload access #28 tests but must not block or alter its source contract.

Validation

• Canonical layout tests for every accepted and rejected niche.
• HIR, MIR, and LLVM tests proving identical behavior for niche and tagged layouts.
• 32-bit and 64-bit ABI coverage.
• Runtime fixtures for none, present values, equality, payload access, and cleanup.