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The compiler_call op + #compiler hook mechanism was fully superseded by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

@@ -336,13 +336,6 @@ fn printInst(instruction: *const Inst, ref_idx: u32, tt: *const TypeTable, write
                 a.clobbers.len,
             });
         },
-        .compiler_call => |cc| {
-            const name = tt.getString(@enumFromInt(cc.name));
-            try writer.print("compiler_call \"{s}\"(", .{name});
-            try writeArgs(cc.args, writer);
-            try writer.writeAll(") : ");
-        },
-
         // ── Closure ─────────────────────────────────────────────
         .closure_create => |cc| {
             try writer.print("closure_create @{d}", .{cc.func.index()});