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fully superseded by abi(.compiler) (P5.5) — no sx code uses them.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/ast.zig

@@ -81,7 +81,6 @@ pub const Node = struct {
         undef_literal: void,
         inferred_type: void,
         builtin_expr: void,
-        compiler_expr: void,
         library_decl: LibraryDecl,
         framework_decl: FrameworkDecl,
         function_type_expr: FunctionTypeExpr,