feat(registry): index cpp/apps/* + e2e test infrastructure

registry/indexer.go ahora escanea <lang>/apps/*/app.md ademas de apps/ y
projects/*/apps/. cpp/apps/chart_demo y cpp/apps/shaders_lab pasan a estar
en registry.db con sus manifests.

Infraestructura de tests e2e (opt-in con -DFN_BUILD_TESTS=ON):
- vendor de Dear ImGui Test Engine (personal/open-source license).
- chart_demo_tests target con tests/chart_demo_tests.cpp.
- /e2e-cpp slash command para crear y ejecutar tests e2e.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

go.mod

@@ -11,7 +11,7 @@ require (
 	github.com/jackc/pgx/v5 v5.9.1
 	github.com/marcboeker/go-duckdb v1.8.5
 	github.com/mattn/go-sqlite3 v1.14.37
-	golang.org/x/crypto v0.48.0
+	golang.org/x/crypto v0.50.0
 	golang.org/x/sync v0.20.0
 	gopkg.in/yaml.v3 v3.0.1
 	nhooyr.io/websocket v1.8.17
@@ -62,6 +62,7 @@ require (
 	go.yaml.in/yaml/v3 v3.0.4 // indirect
 	golang.org/x/exp v0.0.0-20250128182459-e0ece0dbea4c // indirect
 	golang.org/x/mod v0.34.0 // indirect
+	golang.org/x/net v0.53.0 // indirect
 	golang.org/x/sys v0.43.0 // indirect
 	golang.org/x/telemetry v0.0.0-20260311193753-579e4da9a98c // indirect
 	golang.org/x/text v0.36.0 // indirect