Add README with gameplay, controls, architecture, and tuning docs

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

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+# Endless Runner
+
+A 2D side-scrolling endless runner built with Rust and [raylib](https://www.raylib.com/).
+
+![Rust](https://img.shields.io/badge/Rust-2024-orange)
+
+## Gameplay
+
+Run as far as you can on procedurally generated platforms. The world scrolls faster the longer you survive. Stomp enemies for bonus points, collect power-ups, and don't fall off the screen.
+
+**Controls**
+
+| Action | Keys |
+|--------|------|
+| Jump | `Space` / `W` / `↑` |
+| Restart | `Space` / `R` / `Enter` |
+
+### Power-ups
+
+| Icon | Name | Effect |
+|------|------|--------|
+| I | **Invincible** | Immune to side hits for 5 s |
+| J | **Jump Boost** | 1.45× jump height for 6 s |
+| 2 | **2× Score** | Double score gain for 8 s |
+
+### Scoring
+
+- Passive: 80 pts/s (multiplied by any active score effect)
+- Stomp: 100 pt bonus per enemy (also multiplied)
+
+## Building
+
+**Prerequisites:** Rust toolchain, raylib system library, a Wayland compositor.
+
+```sh
+# debug
+cargo run
+
+# release (LTO + stripped)
+cargo run --release
+```
+
+**Tests**
+
+```sh
+cargo test
+```
+
+42 unit tests covering physics, collision, effects, and level generation.
+
+## Architecture
+
+```
+src/
+  main.rs       — entry point, 60 fps cap, dt capped at 0.05 s
+  config.rs     — Config struct (all tunable parameters)
+  player.rs     — vertical physics, coyote-time jump window
+  platform.rs   — scrolling platforms
+  enemy.rs      — scrolling enemies with stomp detection
+  pickup.rs     — PickupDef (data) + in-world Pickup instance
+  effects.rs    — ActiveEffect trait + three concrete impls
+  level_gen.rs  — procedural generation (StdRng, scrolling frontier)
+  world.rs      — simulation loop, collision resolution, rendering
+  game.rs       — state machine (Playing / GameOver), HUD
+```
+
+**Key design decisions**
+
+- Player x is fixed; only the world scrolls.
+- Speed follows `initial + factor × ln(1 + elapsed / time_scale)` — fast early ramp, logarithmic plateau.
+- Landing detection uses the previous and current bottom-y crossing the platform surface, making it tunnelling-safe at high speeds.
+- Coyote time: the jump window stays open for 120 ms after leaving a platform edge.
+- Power-ups are fully data-driven via `Config::pickup_defs`; adding a new type requires one `PickupDef` entry and one `ActiveEffect` impl.
+
+## Tuning
+
+All gameplay parameters live in `Config::default()` in `src/config.rs` — no recompilation of other modules needed when tweaking values.
+
+```rust
+initial_speed: 280.0,       // px/s at t=0
+speed_log_factor: 150.0,    // how much speed grows
+speed_time_scale: 30.0,     // seconds before growth slows
+gravity: 1900.0,            // px/s²
+jump_velocity: -800.0,      // px/s (negative = up)
+coyote_time: 0.12,          // seconds
+enemy_spawn_chance: 0.35,   // per platform
+pickup_spawn_chance: 0.28,  // per platform
+```