feat: add generated scene skill platform hardening

docs/superpowers/specs/2026-04-19-g2-remaining-fail-closed-closure-design.md

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+# G2 Remaining Fail-Closed Closure Design
+
+> Date: 2026-04-19
+> Status: Draft
+> Parent Framework Plan: `docs/superpowers/plans/2026-04-19-scene-skill-102-full-coverage-framework-plan.md`
+> Parent Route: `Route 3: G2 / multi_mode_request`
+> Parent Layer: `Layer C + Layer D`
+
+## Intent
+
+Reduce the remaining `G2 / multi_mode_request` structured fail-closed bucket after Route 2 is complete or deferred.
+
+## Fixed Input Bucket
+
+`multi_mode_request = 4`
+
+The child plan owns only the currently remaining `G2` structured fail-closed scenes.
+
+## Allowed Files
+
+1. `src/generated_scene/analyzer.rs`
+2. `src/generated_scene/generator.rs`
+3. `src/generated_scene/ir.rs`
+4. `tests/scene_generator_test.rs`
+5. route-local Route 3 inventory and report assets
+
+## Forbidden Files
+
+1. `tests/fixtures/generated_scene/scene_execution_board_2026-04-18.json`
+2. Route 2 assets
+3. Route 4+ assets
+
+## Expected Delta
+
+1. reduce the Route 3 bucket count
+2. preserve current real-sample `G2` executed-pass
+
+## Stop Rule
+
+Stop after the Route 3 bucket is rerun and either:
+
+1. reduced, or
+2. explicitly deferred with named blocker
+