feat(llvm): Major refactor - BuilderCursor全域化 & Resolver API導入

Added:
- Resolver API (resolve_i64) for unified value resolution with per-block cache
- llvmlite harness (Python) for rapid PHI/SSA verification
- Comprehensive LLVM documentation suite:
  - LLVM_LAYER_OVERVIEW.md: Overall architecture and invariants
  - RESOLVER_API.md: Value resolution strategy
  - LLVM_HARNESS.md: Python verification harness

Updated:
- BuilderCursor applied to ALL lowering paths (externcall/newbox/arrays/maps/call)
- localize_to_i64 for dominance safety in strings/compare/flow
- NYASH_LLVM_DUMP_ON_FAIL=1 for debug IR output

Key insight: LoopForm didn't cause problems, it just exposed existing design flaws:
- Scattered value resolution (now unified via Resolver)
- Inconsistent type conversion placement
- Ambiguous PHI wiring responsibilities

Next: Wire Resolver throughout, achieve sealed=ON green for dep_tree_min_string

docs/RESOLVER_API.md

@@ -0,0 +1,25 @@
+# Resolver API (Minimal i64 Prototype)
+
+Goals
+- Centralize "ValueId → current-block value" resolution.
+- Guarantee dominance by localizing values at the start of the block (before non-PHI).
+- De-duplicate per (block, value) to avoid redundant PHIs/casts.
+
+Design
+- `Resolver` keeps small per-function caches keyed by `(BasicBlockId, ValueId)`.
+- `resolve_i64(...)` returns an `i64`-typed `IntValue`, inserting PHI and casts as needed using sealed snapshots.
+- Internally uses `flow::localize_to_i64(...)` for now; later, fold logic directly and add `resolve_ptr/resolve_f64`.
+
+Usage (planned wiring)
+- Create `let mut resolver = instructions::Resolver::new();` at function lowering start.
+- Replace all integer value fetches in lowerers with `resolver.resolve_i64(...)`.
+- Keep builder insertion discipline via `BuilderCursor`.
+
+Next
+- Add `resolve_ptr(...)` and `resolve_f64(...)` with same caching discipline.
+- Migrate existing `localize_to_i64` call sites to the resolver.
+- Enforce vmap direct access ban in lowerers (Resolver-only for reads).
+
+Acceptance tie-in
+- Combined with LoopForm: dispatch-only PHI + resolver-based value access → dominance violations drop to zero (A2.5).
+