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fully superseded by abi(.compiler) (P5.5) — no sx code uses them.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

@@ -1294,7 +1294,6 @@ pub const Analyzer = struct {
             .undef_literal,
             .inferred_type,
             .builtin_expr,
-            .compiler_expr,
             .library_decl,
             .framework_decl,
             .function_type_expr,
@@ -1767,7 +1766,6 @@ pub fn findNodeAtOffset(node: *Node, offset: u32) ?*Node {
         .undef_literal,
         .inferred_type,
         .builtin_expr,
-        .compiler_expr,
         .library_decl,
         .framework_decl,
         .function_type_expr,