added rules

.copilot-rules.md

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+# Copilot Rules — Spacetris (SDL3 / C++20)
+
+These rules define **non-negotiable constraints** for all AI-assisted changes.
+They exist to preserve determinism, performance, and architecture.
+
+If these rules conflict with `.github/copilot-instructions.md`,
+**follow `.github/copilot-instructions.md`.**
+
+---
+
+## Project Constraints (Non-Negotiable)
+
+- Language: **C++20**
+- Runtime: **SDL3** + **SDL3_ttf**
+- Build system: **CMake**
+- Dependencies via **vcpkg**
+- Assets must use **relative paths only**
+- Deterministic gameplay logic is mandatory
+
+Do not rewrite or refactor working systems unless explicitly requested.
+
+---
+
+## Repo Layout & Responsibilities
+
+- Core gameplay loop/state: `src/Game.*`
+- Entry point: `src/main.cpp`
+- Text/TTF: `src/Font.*`
+- Audio: `src/Audio.*`, `src/SoundEffect.*`
+- Effects: `src/LineEffect.*`, `src/Starfield*.cpp`
+- High scores: `src/Scores.*`
+- Packaging: `build-production.ps1`
+
+When adding a module:
+- Place it under `src/` (or an established subfolder)
+- Register it in `CMakeLists.txt`
+- Avoid circular includes
+- Keep headers minimal
+
+---
+
+## Build & Verification
+
+Prefer existing scripts:
+
+- Debug: `cmake --build build-msvc --config Debug`
+- Release:
+  - Configure: `cmake -S . -B build-release -DCMAKE_BUILD_TYPE=Release`
+  - Build: `cmake --build build-release --config Release`
+- Packaging (Windows): `./build-production.ps1`
+
+Before finalizing changes:
+- Debug build must succeed
+- Packaging must succeed if assets or DLLs are touched
+
+Do not introduce new build steps unless required.
+
+---
+
+## Coding & Architecture Rules
+
+- Match local file style (naming, braces, spacing)
+- Avoid large refactors
+- Prefer small, testable helpers
+- Avoid floating-point math in core gameplay state
+- Game logic must be deterministic
+- Rendering code must not mutate game state
+
+---
+
+## Rendering & Performance Rules
+
+- Do not allocate memory per frame
+- Do not load assets during rendering
+- No blocking calls in render loop
+- Visual effects must be time-based (`deltaTime`)
+- Rendering must not contain gameplay logic
+
+---
+
+## Threading Rules
+
+- SDL main thread:
+  - Rendering
+  - Input
+  - Game simulation
+- Networking must run in a **separate thread**
+- SDL main loop must **never block** on networking
+- Cross-thread communication via queues or buffers only
+
+---
+
+## Assets, Fonts, and Paths
+
+- Runtime expects adjacent `assets/` directory
+- `FreeSans.ttf` must remain at repo root
+- New assets:
+  - Go under `assets/`
+  - Must be included in `build-production.ps1`
+
+Never hardcode machine-specific paths.
+
+---
+
+## AI Partner (COOPERATE Mode)
+
+- AI is **supportive**, not competitive
+- AI must respect sync timing and shared grid logic
+- AI must not “cheat” or see hidden future pieces
+- AI behavior must be deterministic per seed/difficulty
+
+---
+
+## Networking (COOPERATE Network Mode)
+
+Follow `network_cooperate_multiplayer.md`.
+
+Mandatory model:
+- **Input lockstep**
+- Transmit inputs only (no board state replication)
+
+Determinism requirements:
+- Fixed tick (e.g. 60 Hz)
+- Shared RNG seed
+- Deterministic gravity, rotation, locking, scoring
+
+Technology:
+- Use **ENet**
+- Do NOT use SDL_net or TCP-only networking
+
+Architecture:
+- Networking must be isolated (e.g. `src/network/NetSession.*`)
+- Game logic must not care if partner is local, AI, or network
+
+Robustness:
+- Input delay buffer (4–6 ticks)
+- Periodic desync hashing
+- Graceful disconnect handling
+
+Do NOT implement:
+- Rollback
+- Full state sync
+- Server-authoritative sim
+- Matchmaking SDKs
+- Versus mechanics
+
+---
+
+## Agent Behavior Rules (IMPORTANT)
+
+- Always read relevant markdown specs **before coding**
+- Treat markdown specs as authoritative
+- Do not invent APIs
+- Do not assume external libraries exist
+- Generate code **file by file**, not everything at once
+- Ask before changing architecture or ownership boundaries
+
+---
+
+## When to Ask Before Proceeding
+
+Ask the maintainer if unclear:
+- UX or menu flow decisions
+- Adding dependencies
+- Refactors vs local patches
+- Platform-specific behavior