V-375: Reuse dependency semantic snapshots for app edits

[drew-y]

Summary

• Add compiler-owned dependency semantic snapshots for std/pkg modules and reuse them across app-source edits.
• Clone arena/effect/typing state safely so app compiles get fresh overlay state without mutating cached dependency semantics.
• Keep profiling and benchmark harness, now measuring real one-line app code edits instead of unchanged artifact hits.

Validation

• npm run typecheck
• npm test
• VOYD_COMPILER_LATENCY_REPEATS=2 VOYD_COMPILER_LATENCY_VTRACE_RUNTIME_SAMPLES=0 VOYD_BENCH_OPTIMIZE_MODES=true npm run bench:compiler-latency
• Focused snapshot/sdk vitest suite: dependency snapshots, std/pkg invalidation, app edits, mixed-order SCCs, generic-heavy vtrace warm edit

Notes

• Warm optimized app-edit compiles save ~10.9% to 27.9% in the recorded scenarios; vtrace remains dominated by whole-program emit/codegen.
• Linear: V-375

[linear]

V-375

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[chatgpt-codex-connector]

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Reviewed commit: ae7a99cb1a

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