P5.7 Step C: delete interp.zig — the comptime VM is the sole evaluator

The legacy tagged-Value Interpreter is gone. Relocate the Value result-DTO
+ decodeVariantElements into a new comptime_value.zig (the VM<->host
materialization boundary); repoint comptime_vm/emit_llvm/ir-barrel Value to
it and BuildConfig to compiler_hooks; delete the dead valueToReg bridge;
slim compiler_lib.zig to just the name registry (BoundFn{sx_name} + bound_fns
+ findFn — weldedCompilerFn only validates names); simplify printInterpBailDiag
to comptime_vm.last_bail_reason; drop the unused interp_mod import in lower.zig.
rm src/ir/interp.zig + interp.test.zig.

Value is relocated (not eliminated): it survives only as the slim result DTO
at the VM->valueToLLVMConst boundary; the execution-time marshaling the VM
pivot targeted is gone. Drop dead Value.asString/reflectTypeId.

706/0 corpus + 476/476 unit.

src/ir/lower.zig

@@ -8,7 +8,6 @@ const mod_mod = @import("module.zig");
 const type_bridge = @import("type_bridge.zig");
 const unescape = @import("../unescape.zig");
 const parser_mod = @import("../parser.zig");
-const interp_mod = @import("interp.zig");
 const errors = @import("../errors.zig");
 const jni_descriptor = @import("jni_descriptor.zig");
 const program_index_mod = @import("program_index.zig");