P5.7 Step B1: remove the compiler_call IR op + the hook Registry

The compiler_call op + #compiler hook mechanism was fully superseded by
abi(.compiler) VM-native dispatch (P5.5) — no sx code emits it anymore.

Remove: the compiler_call op variant + CompilerCall struct (inst.zig); the
Builder.compilerCall emitter (module.zig); the two dead producer blocks in
lower/call.zig (compiler_expr-bodied free fns + methods); every consumer
switch arm (emit_llvm, ops.emitCompilerCall, print, interp dispatch); the
interp.hooks field + init/deinit. Strip compiler_hooks.zig down to the still-
live BuildConfig / BuildHooks / AssetDir (delete HookError/HookFn/Registry/
registerDefaults + all hookXxx, and the now-unused interp/Value imports).

Test refs that used compiler_call as a sample unported op now use vec_splat.

501/501 unit + 706/0 corpus.

src/backend/llvm/ops.zig

@@ -1459,11 +1459,6 @@ pub const Ops = struct {
         }
     }
 
-    pub fn emitCompilerCall(self: Ops, instruction: *const Inst) void {
-        // Compiler hooks are comptime-only; if one reaches emission, produce undef
-        self.e.mapRef(c.LLVMGetUndef(self.e.toLLVMType(instruction.ty)));
-    }
-
     pub fn emitCallClosure(self: Ops, instruction: *const Inst, call_op: CallIndirect) void {
         // Closure: { fn_ptr, env }.
         //