P5.7 Step B2: remove the #compiler attribute + compiler_expr AST node

The #compiler struct attribute + #compiler-suffixed bodyless methods were
fully superseded by abi(.compiler) (P5.5) — no sx code uses them.

Remove the hash_compiler token (token/lexer/lsp), the is_compiler_struct /
struct_default_compiler parser machinery + the two compiler_expr body-
synthesis branches, the compiler_expr AST variant, and every
.builtin_expr/.compiler_expr switch arm + == .compiler_expr check across
sema/resolver/semantic_diagnostics/generic/decl/call/calls (kept .builtin_expr).
abi(.compiler) is untouched. Delete the obsolete calls.test.zig dispatch test.

500/500 unit + 706/0 corpus.

src/ir/resolver.zig

@@ -184,7 +184,7 @@ pub fn isPlainFreeFnDecl(fd: *const ast.FnDecl) bool {
     // body, so it stays plain-free.
     if (fd.extern_export == .extern_) return false;
     return switch (fd.body.data) {
-        .builtin_expr, .compiler_expr => false,
+        .builtin_expr => false,
         else => true,
     };
 }