feat: Add comprehensive roadmap for JSON configuration and rendering engine

ROADMAP.md

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+# ROADMAP
+
+## North Star
+Treat JSON config as a declarative control plane that compiles into scene, resource, and render graphs with strict validation, budget enforcement, and crash-resistant policies.
+
+## Guiding Principles
+- Fail fast with clear JSON-path error reporting.
+- Keep APIs explicit, predictable, and easy to reason about.
+- Prefer refactoring that reduces boilerplate and hardcoded state.
+
+## Status Legend
+- [x] live now
+- [~] partial / limited
+- [ ] planned
+
+## Launch Packages (Cheesy Edition)
+
+### Starter Plan: "Bootstrap Hosting"
+- [x] Config version gating (`schema_version` / `configVersion` checks)
+- [ ] JSON Schema validation (external schema + validator)
+- [~] JSON-path diagnostics (path strings in exceptions, not full JSON Pointer coverage)
+- [~] Layered config merges (supports `extends` + `@delete`; no profile/local/CLI yet)
+- [x] Trace logging around config load, validation, and merge steps
+- [ ] Migration stubs for future versions
+
+### Pro Plan: "Graph Builder"
+- [ ] Typed IRs: `SceneIR`, `ResourceIR`, `RenderGraphIR`
+- [ ] Symbol tables + reference resolution with clear diagnostics
+- [ ] Render graph DAG compile with cycle detection
+- [ ] "Use before produce" validation for render pass inputs
+- [ ] Explicit pass scheduling and backend submission planning
+
+### Ultra Plan: "Probe Fortress"
+- [ ] Probe hooks: `OnLoadScene`, `OnCreatePipeline`, `OnDraw`, `OnPresent`, `OnFrameEnd`
+- [x] Pipeline compatibility checks (mesh layout vs shader inputs) via shader pipeline validator
+- [x] Sampler limits enforced from bgfx caps
+- [ ] Shader uniform compatibility enforcement
+- [~] Resource budget enforcement (texture memory + max texture dim + GUI caches)
+- [x] Crash recovery service (heartbeats, GPU hang detection, memory monitoring)
+- [ ] Probe severity mapped to crash recovery policies
+
+### Enterprise Plan: "Demo Deluxe"
+- [ ] Service refactors to reduce boilerplate and hardcoded state
+- [ ] JSON-driven component tree generation (entities, materials, passes)
+- [ ] Cube demo rebuilt on config-first scene + render pipeline
+- [ ] Hot-reload with diff logging and rollback on validation failure
+- [ ] Configurable feature flags to isolate subsystems quickly
+
+## Feature Matrix (What You Get, When You Get It)
+
+| Feature | Status | Starter | Pro | Ultra | Enterprise |
+| --- | --- | --- | --- | --- | --- |
+| Config version gating (`schema_version` / `configVersion`) | Live | [x] | [ ] | [ ] | [ ] |
+| JSON Schema validation | Planned | [x] | [ ] | [ ] | [ ] |
+| Layered config merges + deterministic rules | Partial | [x] | [ ] | [ ] | [ ] |
+| JSON-path diagnostics | Partial | [x] | [ ] | [ ] | [ ] |
+| IR compilation (scene/resources/render) | Planned | [ ] | [x] | [ ] | [ ] |
+| Render graph DAG build + cycle checks | Planned | [ ] | [x] | [ ] | [ ] |
+| Pass scheduling + submission planning | Planned | [ ] | [x] | [ ] | [ ] |
+| Probe system + structured reports | Planned | [ ] | [ ] | [x] | [ ] |
+| Pipeline compatibility checks | Live | [ ] | [ ] | [x] | [ ] |
+| Sampler limits enforced | Live | [ ] | [ ] | [x] | [ ] |
+| Shader uniform compatibility enforcement | Planned | [ ] | [ ] | [x] | [ ] |
+| Budget enforcement + fallback policies | Partial | [ ] | [ ] | [x] | [ ] |
+| Crash recovery integration | Live | [ ] | [ ] | [x] | [ ] |
+| JSON-driven component trees | Planned | [ ] | [ ] | [ ] | [x] |
+| Cube demo upgrade | Planned | [ ] | [ ] | [ ] | [x] |
+| Hot-reload + rollback | Planned | [ ] | [ ] | [ ] | [x] |
+
+## Deliverables Checklist
+- [ ] `config/schema/` with versioned JSON Schema and migration notes
+- [ ] `src/services/impl/config_compiler_service.*` for JSON -> IR compilation
+- [ ] `src/services/impl/render_graph_service.*` for graph build and scheduling
+- [ ] `src/services/interfaces/i_probe_service.hpp` plus report/event types
+- [~] Budget enforcement with clear failure modes and fallback resources (textures + GUI caches today)
+- [ ] Cube demo config-only boot path
+
+## Tests and Verification Checklist
+- [ ] Unit tests for schema validation, reference resolution, and merge rules
+- [ ] Graph validation tests for cycles and invalid dependencies
+- [x] Pipeline compatibility tests (shader inputs vs mesh layouts)
+- [ ] Budget enforcement tests (over-limit textures, transient pool caps)
+- [ ] Smoke test: cube demo boots with config-first scene definition
+
+## Open Questions
+- Preferred merge behavior for array fields (replace vs keyed merge by `id`)
+- Scope of hot-reload (full scene reload vs incremental updates)
+- Target shader reflection source (bgfx, MaterialX, or custom metadata)