astgen: catch error capture, @panic, @errorName, @field, larger block scopes

- Add error capture scope to orelseCatchExpr (catch |err| now creates
  a ScopeLocalVal for the captured error variable)
- Add @panic, @errorName, @field builtins
- Increase blockExprStmts scope arrays from 64 to 128 entries
  (build.zig has 93 var decls in a single block)

corpus build.zig still skipped: needs switchExprErrUnion optimization
(catch |err| switch(err) pattern).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

stage0/astgen_test.zig

@@ -793,6 +793,9 @@ test "astgen: corpus test_all.zig" {
 }
 
 test "astgen: corpus build.zig" {
+    // TODO: 6 extra instructions — missing switchExprErrUnion optimization
+    // (catch |err| switch(err) pattern emits SWITCH_BLOCK instead of
+    // SWITCH_BLOCK_ERR_UNION).
     if (true) return error.SkipZigTest;
     const gpa = std.testing.allocator;
     try corpusCheck(gpa, @embedFile("../build.zig"));