docs: add gol-simulation OpenSpec change

Plan Game of Life mechanics: Conway rules, pause/resume, stepping,
speed control, and paused-only cell editing with drag painting.

openspec/changes/gol-simulation/design.md

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+# Design
+
+## Context
+
+Input, placement, HiDPI, and grid are implemented. Simulation state (`sim_paused`, `ACTION_PAUSE`, `ACTION_STEP`, speed actions) exists but is unused. See proposal.md.
+
+## Goals / Non-Goals
+
+**Goals:**
+- Conway tick loop with adjustable interval.
+- Pause/resume, stepping, speed control.
+- Paused-only cell editing via mouse, including drag painting.
+
+**Non-Goals:**
+- Pattern detection or statistics.
+- Undo/redo.
+- Multi-threading.
+
+## Decisions
+
+### D1: Tick timing
+- Use monotonic clock in the frame loop; accumulate elapsed time; when >= interval, compute next generation and reset accumulator.
+
+### D2: Next generation buffer
+- Allocate a second grid buffer; compute into it, then swap pointers (or copy). Prefer swap of `cells` pointers with size unchanged.
+
+### D3: Editing while paused
+- Pointer button handler sets/clears cells if `sim_paused`; drag paints using motion events while button held.
+
+### D4: Speed bounds
+- Interval range 0.05s..1.0s (20 tps..1 tps), step factor 2.
+
+### D5: Default bindings
+- Add `p`, `n`, `+`/`-` to default config (merge).
+
+## Risks / Trade-offs
+
+- [Tick cost on large grids] -> grid is small (50x50), fine.
+- [Editing during tick race] -> editing only while paused avoids races.
+
+## Open Questions
+
+None.