Phase 11-12: LLVM backend initial, semantics layer, plugin unification

Major changes:
- LLVM backend initial implementation (compiler.rs, llvm mode)
- Semantics layer integration in interpreter (operators.rs)
- Phase 12 plugin architecture revision (3-layer system)
- Builtin box removal preparation
- MIR instruction set documentation (26→Core-15 migration)
- Cross-backend testing infrastructure
- Await/nowait syntax support

New features:
- LLVM AOT compilation support (--backend llvm)
- Semantics layer for interpreter→VM flow
- Tri-backend smoke tests
- Plugin-only registry mode

Bug fixes:
- Interpreter plugin box arithmetic operations
- Branch test returns incorrect values

Documentation:
- Phase 12 README.md updated with new plugin architecture
- Removed obsolete NYIR proposals
- Added LLVM test programs documentation

Co-Authored-By: Claude <noreply@anthropic.com>

docs/issues/llvm_binop_string_mismatch.md

@@ -0,0 +1,40 @@
+# LLVM lowering: string + int causes binop type mismatch
+
+Status: open
+
+Summary
+
+- When compiling code that concatenates a string literal with a non-string (e.g., integer), LLVM object emission fails with a type mismatch in binop.
+- Example from `apps/ny-map-llvm-smoke/main.nyash`: `print("Map: v=" + v)` and `print("size=" + s)`.
+
+Environment
+
+- LLVM 18 / inkwell 0.5.0
+- Phase 11.2 lowering
+
+Repro
+
+1) Run: `NYASH_LLVM_ARRAY_SMOKE=1 ./tools/llvm_smoke.sh release` (or build/link the map smoke similarly)
+2) Observe: `❌ LLVM object emit error: binop type mismatch`
+
+Expected
+
+- String concatenation should be lowered to a safe runtime shim (e.g., NyRT string builder or `nyash_string_concat`) that accepts `(i8* string, i64/int)` and returns `i8*`.
+
+Observed
+
+- `+` binop is currently generated as integer addition for non-float operands, leading to a type mismatch when one side is a pointer (string) and the other is integer.
+
+Plan
+
+1) Introduce string-like detection in lowering: if either operand is `String` (or pointer from `nyash_string_new`), route to a NyRT concat shim.
+2) Provide NyRT APIs:
+   - `nyash.string.concat_ss(i8*, i8*) -> i8*`
+   - `nyash.string.concat_si(i8*, i64) -> i8*`
+   - Optional: `concat_sf`, `concat_sb` (format helpers)
+3) As an interim simplification for smoke, emit `print("..." )` in two steps to avoid mixed-type `+` until the concat shim is ready.
+
+CI
+
+- Keep `apps/ny-llvm-smoke` OFF by default. Re-enable once concat shim lands and binop lowering is updated.
+