comptime VM: flat-memory machine + executor + Reg<->Value bridge + tryEval

Phase 1 of the flat-memory comptime VM (current/PLAN-COMPILER-VM.md),
built standalone + unit-tested with the legacy interpreter still live and
the corpus untouched (688 green).

src/ir/comptime_vm.zig:
- Machine: one linear byte memory (comptime stack+heap) with a bump/stack
  allocator (mark/reset), scalar readWord/writeWord (1/2/4/8 LE) + byte
  views; addr 0 reserved as null_addr. Frame: a Ref-indexed register file
  (Reg = raw u64: immediate scalar bits OR an Addr). Target-aware layout
  comes from the type table, so cross-compilation stays correct.
- Vm executor over the SAME SSA IR, mirroring the legacy interp's scalar
  semantics (i64 wrapping/signed, f64). Ported: constants, arithmetic,
  comparison, logical, conversions, control flow (br/cond_br/ret + block
  params); structs (alloca/load/store/struct_init/get/gep at target
  offsets); tuples; arrays (index_get/gep, length); slices+strings as
  {ptr,len} fat pointers (const_string, data_ptr, subslice,
  array_to_slice, str_eq/ne, index-through-slice); optionals (pointer and
  {T,i1} shapes); payloadless enums; deref/addr_of; direct + recursive
  call over the shared flat memory (depth-guarded). The value model: a
  word for scalars/pointers, by-address for aggregates (a struct's value
  IS its Addr). Any unported op bails loudly (error.Unsupported + detail).
- Reg<->Value boundary bridge (valueToReg / regToValue) + tryEval, the
  hybrid-wiring entry point: run a comptime fn on the VM, return a legacy
  Value or null to fall back. Transitional, for the legacy interop edge.

Registered in the ir.zig barrel.

src/ir/ir.zig

@@ -61,6 +61,7 @@ pub const ErrorFacts = error_analysis.ErrorFacts;
 
 pub const compiler_hooks = @import("compiler_hooks.zig");
 pub const compiler_lib = @import("compiler_lib.zig");
+pub const comptime_vm = @import("comptime_vm.zig");
 pub const emit_llvm = @import("emit_llvm.zig");
 pub const LLVMEmitter = emit_llvm.LLVMEmitter;
 
@@ -91,6 +92,7 @@ pub const emit_llvm_tests = @import("emit_llvm.test.zig");
 pub const jni_descriptor_tests = @import("jni_descriptor.test.zig");
 pub const jni_java_emit_tests = @import("jni_java_emit.test.zig");
 pub const compiler_lib_tests = @import("compiler_lib.test.zig");
+pub const comptime_vm_tests = @import("comptime_vm.test.zig");
 
 test {
     @import("std").testing.refAllDecls(@This());