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/ir.zig

@@ -2,7 +2,7 @@ pub const types = @import("types.zig");
 pub const inst = @import("inst.zig");
 pub const module = @import("module.zig");
 pub const print = @import("print.zig");
-pub const interp = @import("interp.zig");
+pub const comptime_value = @import("comptime_value.zig");
 pub const lower = @import("lower.zig");
 pub const program_index = @import("program_index.zig");
 pub const resolver = @import("resolver.zig");
@@ -40,8 +40,7 @@ pub const Builder = module.Builder;
 pub const ImplTable = module.ImplTable;
 
 pub const printModule = print.printModule;
-pub const Interpreter = interp.Interpreter;
-pub const Value = interp.Value;
+pub const Value = comptime_value.Value;
 pub const Lowering = lower.Lowering;
 pub const ProgramIndex = program_index.ProgramIndex;
 pub const TypeResolver = type_resolver.TypeResolver;
@@ -75,7 +74,6 @@ pub const types_tests = @import("types.test.zig");
 pub const inst_tests = @import("inst.test.zig");
 pub const module_tests = @import("module.test.zig");
 pub const print_tests = @import("print.test.zig");
-pub const interp_tests = @import("interp.test.zig");
 pub const lower_tests = @import("lower.test.zig");
 pub const program_index_tests = @import("program_index.test.zig");
 pub const resolver_tests = @import("resolver.test.zig");