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fully superseded by abi(.compiler) (P5.5) — no sx code uses them.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

@@ -118,7 +118,6 @@ pub const Tag = enum {
     hash_import, // #import
     hash_insert, // #insert
     hash_builtin, // #builtin
-    hash_compiler, // #compiler
     hash_library, // #library
     hash_framework, // #framework
     hash_using, // #using