Fix string ABI and place traversal duplication

[itsfuad]

Summary

• Keep @expr as one direct raw-address operation through MIR and LLVM place pointers.
• Centralize borrowed/owned string carrier extraction and preserve owned allocator extraction.
• Reject borrowed-string comparisons before scalar LLVM lowering.
• Centralize selector/index place traversal and add direct place-policy coverage.

Validation

• CCACHE_DISABLE=1 GOCACHE=/tmp/peeper-go-cache go test -count=1 ./...
• CCACHE_DISABLE=1 GOCACHE=/tmp/peeper-go-cache go build ./...
• CCACHE_DISABLE=1 GOCACHE=/tmp/peeper-go-cache go vet ./...
• Focused place, typechecker, ownership, HIR, MIR, and LLVM tests
• Bundled compiler and changed positive/negative/trap fixtures
• gofmt and git diff --check

Tracking

Progresses #29 (Define string runtime semantics). Remaining C/FFI boundary and runtime-construction work stays open under #29.

Stack

This is stack base. Follow-ups are #69 (32-bit ABI/intrinsics), #70 (contract spine), #71 (end-to-end remediation), and #72 (open-ended default range cloning).