docs: contests skip AST check in Phase 1

Add contest behavior section: AST check is skipped when contest_id
is set. Contest AST support with enable_ast_check flag deferred to
Phase 2.

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

docs/specs/2026-05-25-ast-checker-design.md

@@ -263,8 +263,9 @@ The AST check happens AFTER the judge server returns a result, and ONLY when the
 # (after _compute_statistic_info and result determination)
 
     # --- AST CHECK (NEW) ---
+    # Skip AST check in contests (Phase 1). Contest AST support deferred to Phase 2.
     # Only check AST when the submission would be AC
-    if self.submission.result == JudgeStatus.ACCEPTED:
+    if self.submission.result == JudgeStatus.ACCEPTED and not self.contest_id:
         ast_rules = self.problem.ast_rules
         if ast_rules and language in ast_rules:
             from ast_checker.checker import check_ast
@@ -484,6 +485,17 @@ One Django migration:
 
 Both are additive, no data migration needed. Existing problems get `ast_rules=null` (no AST checking).
 
+### Contest Behavior
+
+**Phase 1: Contests skip AST check entirely.** Even if a contest problem has `ast_rules` configured, submissions during a contest are not AST-checked. The guard is a single condition: `not self.contest_id`.
+
+Reasons:
+- Contests focus on correctness and speed; syntax enforcement is a practice/homework tool
+- Showing "代码检查未通过" in a timed contest would confuse contestants (the submission IS correct)
+- Avoids complexity around ACM penalty time and OI score calculation for AST_CHECK_FAILED
+
+**Phase 2 (deferred):** Add a `enable_ast_check` boolean on the Contest model, allowing contest creators to opt in. If enabled, AST_CHECK_FAILED counts as AC for ranking, penalty, and score — same semantics as regular problems.
+
 ### Legacy Data Policy
 
 - **Existing submissions are not retroactively checked.** When a teacher adds AST rules to an existing problem, only new submissions are AST-checked. Prior AC submissions remain AC.