docs: add legacy data policy to AST checker spec

Existing submissions are not retroactively checked. AST re-check
for existing AC submissions deferred to Phase 2.

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

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

@@ -381,10 +381,16 @@ These are pure Python wheels with pre-compiled grammars, no system dependencies
 
 One Django migration:
 1. Add `ast_rules` JSONField (null=True) to Problem model
-2. Add `AST_CHECK_FAILED = 9` to JudgeStatus
+2. Add `AST_CHECK_FAILED = 10` to JudgeStatus
 
 Both are additive, no data migration needed. Existing problems get `ast_rules=null` (no AST checking).
 
+### 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.
+- **No data migration required.** `accepted_number` and `statistic_info` keep their current values. The `statistic_info` will naturally accumulate `"10"` entries as new AST_CHECK_FAILED submissions come in.
+- **Phase 2: optional "AST re-check"** — an admin action to re-run AST rules on all existing AC submissions for a given problem. Not in Phase 1.
+
 ---
 
 ## Phased Delivery