refactor(v4): split MIR construction from representation

[OttoApocalypse69]

What changed

• Split V4 MIR into freak_mir for persistent Built-MIR representation, validation, and snapshot v5, and freak_mir_build for HIR+TY-to-MIR construction policy and request-scoped scratch.
• Preserve all 610 original MIR tasks, public v4_mir_lower_ty, allocation order, query/driver/editor/snapshot behavior, and byte-identical snapshot/restore task bodies.
• Move Meiya-specific returned-loan interpretation into freak_borrowck; borrowck and LLVM no longer call builder-owned tasks.
• Add architecture guards for downstream builder consumption and an exact shrinking allowlist for transitional syntax dependencies.

Why this boundary

freak_mir owns the stable Built-MIR data model. freak_mir_build owns source-facing lowering policy. This is the mechanical predecessor for No-Syntax-Past-HIR slices and freak_mir_transform; it deliberately does not redesign MIR or version snapshot v5.

Validation

Exact head: 5928d4ad685fe47053b8cb10f9c717f31ee3599b
Base: fix/v4-post-merge-contract-hardening@d26e2e79be8883eabdaf26ad71af6d59b0e7236b (#91)

• Python compilation and conformance audit pass.
• 20 focused executable smokes pass.
• check_v4.py --fast: all 247 fixtures, 3,618 statements, 39.5 MB peak, exit 0.
• Clean immutable diff.
• Independent exact-head review: APPROVE, no P0-P3 findings.

Pending

Current-head CI and automated review on this stacked draft PR.

Conservative boundaries

• Transitional lexer/parser/token dependencies in freak_mir_build are documented and exactly pinned for later shrinkage.
• Meiya derives returned-loan call provenance from Built MIR plus TY facts; persisting it requires a deliberate versioned snapshot slice.
• Pre-existing token-facing editor helpers remain a separate tooling-boundary slice.

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