[codex] Inline Uint8Array access for const local lengths

[andrewtdiz]

Summary

• Teach the noalias buffer fact collector to remember prior immutable numeric length locals.
• Allow const buf = new Uint8Array(SIZE) when SIZE is an immutable finite integer length to register a native-owned Uint8Array view.
• Reuse the existing bounded-loop proof so buf[i] lowers to inline byte load/store instead of js_uint8array_get/set, with collector and IR regression coverage.

Verification

• cargo fmt --check
• git diff --check
• cargo test -p perry-codegen collectors::hir_facts -- --nocapture (9 passed)
• cargo test -p perry-codegen --test native_proof_buffer_views -- --nocapture (22 passed)
• cargo test -p perry-codegen --test native_proof_regressions -- --nocapture (45 passed)
• cargo build --release -p perry
• target/release/perry compile benchmarks/suite/bench_buffer_readwrite.ts -o /tmp/perry-cycle18-final-bench_buffer_readwrite --trace llvm
• IR evidence from /tmp/perry_llvm_272534_1781722418994040407_0.ll: hot path contains store i8 and load i8; no matched call i32 @js_uint8array_get or call void @js_uint8array_set.

Benchmark

bench_buffer_readwrite, checksum 12749385600 for all runs:

• Baseline /tmp/perry-cycle18-baseline-bench_buffer_readwrite: 836, 824, 817, 864, 832 ms; avg 834.6 ms.
• Final /tmp/perry-cycle18-final-bench_buffer_readwrite: 94, 86, 76, 99, 76 ms; avg 86.2 ms.
• Speedup: 9.7x, about 89.7% lower runtime.

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