feat: add explicit region materialization

[sanohiro]

Summary

• add named arena capabilities and explicit clone_in(region) materialization
• add region-backed array_builder<T> support for recursively RegionPlain values across whole-program and per-unit compilation
• preserve concrete aggregate element types, caller region provenance, and borrow-mut builder ownership across HIR, MIR, interfaces, LLVM lowering, and runtime storage
• extend checked-HIR and canonical interface validation for the new capability and builder forms

Why this capability is one PR

The producer and consumer form one strict ownership/ABI chain: region capabilities, materialization, aggregate builder descriptors, interface serialization, LLVM layout, and runtime allocation are not independently useful dormant seams. Keeping them together avoids duplicated cross-stage proofs and tests while leaving one mergeable end-to-end capability.

Review fixes

The bounded full-diff review found four issues, all closed in f5cbd414:

• include named arenas in every MoveCheck transparent-block path
• reject clone_in lengths above the target isize::MAX
• keep the 64-byte array-builder stack storage as an ABI envelope on 32-bit targets
• preserve zero-sized RegionPlain element counts in region builders

Validation

• scripts/pre-pr.sh on final SHA f5cbd414 with the preceding review findings recorded as fixed
• scripts/test-pr.sh
• scripts/cargo.sh clippy --workspace --lib --bins --locked -- -D warnings
• align_driver fb_region: 36 passed
• align_sema unit suite: 207 passed
• align_runtime region-array-builder owners: 4 passed