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The compiler_call op + #compiler hook mechanism was fully superseded by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/ir/emit_llvm.zig

@@ -1632,7 +1632,6 @@ pub const LLVMEmitter = struct {
 
             // ── Call extensions ───────────────────────────────────────
             .call_builtin => |bi| self.ops().emitCallBuiltin(instruction, bi),
-            .compiler_call => self.ops().emitCompilerCall(instruction),
             .call_closure => |call_op| self.ops().emitCallClosure(instruction, call_op),
 
             // ── Tuple ops ────────────────────────────────────────────