Add public API boundary checks

docs/roadmap/implementation.md

@@ -481,7 +481,7 @@ guidance and documented the command contract.
 
 ## Stage 3: Protect public contracts and dependency direction
 
-Status: Pending. Depends on Stage 2.
+Status: Complete. Depends on Stage 2.
 
 1. Add a compiled-export boundary checker for both facades using `go list` export
    data and `go/types`. Inspect exported signatures, fields, callbacks, aliases,
@@ -499,6 +499,15 @@ Acceptance: actual exports pass and meaningful negative fixtures fail. Tests
 inspect contracts and imports, with no fixed inventory of future internal
 package names. Extend these checks as subsequent exports land.
 
+Completion evidence: added `internal/apicheck`, which loads compiled exports
+from `go list -deps -export` and inspects signatures, fields, aliases, generic
+constraints, and pointer and value method sets through `go/types`. The checks
+pass for both public facades and enforce internal import direction and the
+runtime dependency boundary. Negative fixtures detect leaked types, aliases,
+callbacks, generic constraints, and promoted implementation methods; a private
+implementation field and selected standard-library contracts pass. Added the
+checker command and internal package inventory to the canonical developer docs.
+
 ## Stage 4: Implement profile loading and command adaptation
 
 Status: Pending. Depends on Stage 3.