fix(drift): scope anchor-presence check to jsonc fenced blocks

Adds `anchor_in_jsonc_block(md, anchor) -> bool` to
`design_schema_drift.rs` — a line-walking helper that toggles on
``` / ~~~ fence markers (any info-string: jsonc, json, or
unspecified — all count) and returns true only if the anchor
appears inside fenced content. Re-routes the seven
`DATA_MODEL.contains(anchor)` assertion sites in the file
(Term, Pattern, Type, Literal, Def, ParamMode, nested struct
keys) onto it.

The carrier from `debug` named six sites; in the implement phase
the orchestrator flagged a seventh — `design_md_anchors_nested_
struct_keys` — that exhibited the identical scope-widening bug and
was structurally identical to the named six. Routing it uniformly
is the cohesive shape of the fix; leaving it as `.contains()`
would have preserved a known false-positive surface inside the
same file the fix targets. The "minimal fix, no surrounding
cleanup" constraint blocks opportunistic refactor, not the
uniform application of the named one-line substitution.

Verified: 8/8 in `design_schema_drift` green (was 7 + 1 compile-
blocking RED at c8c30d5); full ailang-core suite green; full
workspace `cargo test` green, no regressions.

Mirrors the inverse `strip_fences` pattern in
`crates/ailang-core/tests/design_index_pin.rs`.

closes #10

bench/orchestrator-stats/2026-05-20-iter-bugfix-drift-anchor-scope.json

@@ -0,0 +1,12 @@
+{
+  "iter_id": "bugfix-drift-anchor-scope",
+  "date": "2026-05-20",
+  "mode": "mini",
+  "outcome": "DONE",
+  "tasks_total": 1,
+  "tasks_completed": 1,
+  "reloops_per_task": { "1": 0 },
+  "review_loops_spec": 0,
+  "review_loops_quality": 0,
+  "blocked_reason": null
+}

crates/ailang-core/tests/design_schema_drift.rs

@@ -21,6 +21,31 @@ use ailang_core::ast::{
 
 const DATA_MODEL: &str = include_str!("../../../design/contracts/data-model.md");
 
+/// Scoped substring match: returns `true` iff `anchor` appears inside a
+/// fenced code block (``` or ~~~) of `md`. Info-string `jsonc` / `json` /
+/// unspecified all count — fence-toggling is by fence-marker line alone,
+/// mirroring the inverse `strip_fences` pattern in
+/// `crates/ailang-core/tests/design_index_pin.rs`. The whole-file
+/// `.contains()` was fidelity-widened: anchors mentioned in surrounding
+/// prose (footnotes, inline references, historical notes) counted as
+/// present, so a future edit could delete the canonical schema entry for
+/// a variant from inside a fenced block and the drift test would still
+/// pass. This helper closes that surface.
+fn anchor_in_jsonc_block(md: &str, anchor: &str) -> bool {
+    let mut in_fence = false;
+    for line in md.lines() {
+        let t = line.trim_start();
+        if t.starts_with("```") || t.starts_with("~~~") {
+            in_fence = !in_fence;
+            continue;
+        }
+        if in_fence && line.contains(anchor) {
+            return true;
+        }
+    }
+    false
+}
+
 /// Every `Term` variant must have its JSON-schema anchor present in
 /// design/contracts/data-model.md. An LLM author cannot produce a term variant
 /// whose `"t"` tag is absent from the canonical schema document.
@@ -156,7 +181,7 @@ fn design_md_anchors_every_term_variant() {
             Term::Recur { .. } => "recur",
         };
         assert!(
-            DATA_MODEL.contains(anchor),
+            anchor_in_jsonc_block(DATA_MODEL, anchor),
             "design/contracts/data-model.md is missing anchor `{anchor}` for a Term variant — \
              add it to design/contracts/data-model.md"
         );
@@ -186,7 +211,7 @@ fn design_md_anchors_every_pattern_variant() {
             Pattern::Ctor { .. } => "ctor",
         };
         assert!(
-            DATA_MODEL.contains(anchor),
+            anchor_in_jsonc_block(DATA_MODEL, anchor),
             "design/contracts/data-model.md is missing anchor `{anchor}` for a Pattern variant"
         );
     }
@@ -219,7 +244,7 @@ fn design_md_anchors_every_type_variant() {
             Type::Forall { .. } => "forall",
         };
         assert!(
-            DATA_MODEL.contains(anchor),
+            anchor_in_jsonc_block(DATA_MODEL, anchor),
             "design/contracts/data-model.md is missing anchor `{anchor}` for a Type variant"
         );
     }
@@ -248,7 +273,7 @@ fn design_md_anchors_every_literal_variant() {
             Literal::Float { .. } => "float",
         };
         assert!(
-            DATA_MODEL.contains(anchor),
+            anchor_in_jsonc_block(DATA_MODEL, anchor),
             "design/contracts/data-model.md is missing anchor `{anchor}` for a Literal variant"
         );
     }
@@ -320,7 +345,7 @@ fn design_md_anchors_every_def_kind() {
             Def::Instance(_) => "instance",
         };
         assert!(
-            DATA_MODEL.contains(anchor),
+            anchor_in_jsonc_block(DATA_MODEL, anchor),
             "design/contracts/data-model.md is missing anchor `{anchor}` for a Def kind"
         );
     }
@@ -344,7 +369,7 @@ fn design_md_anchors_every_parammode_variant() {
             ParamMode::Borrow => "borrow",
         };
         assert!(
-            DATA_MODEL.contains(anchor),
+            anchor_in_jsonc_block(DATA_MODEL, anchor),
             "design/contracts/data-model.md is missing anchor `{anchor}` for a ParamMode variant"
         );
     }
@@ -457,7 +482,7 @@ fn design_md_anchors_nested_struct_keys() {
 
     for anchor in anchors {
         assert!(
-            DATA_MODEL.contains(anchor),
+            anchor_in_jsonc_block(DATA_MODEL, anchor),
             "design/contracts/data-model.md is missing nested-struct-key anchor `{anchor}`"
         );
     }