docs: add arena-size OpenSpec change

Plan output-aware arena sizing (~80% of screen) with fallback and resize.
2026-11-08 16:05:14 +01:00
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+# Design
+
+## Context
+
+Output is already bound and a listener exists, but mode dimensions are ignored. See proposal.md.
+
+## Goals / Non-Goals
+
+**Goals:**
+- Store output mode width/height.
+- Compute default logical size = 0.8 * mode / scale.
+- Keep grid resize/preserve behavior.
+
+**Non-Goals:**
+- Fullscreen mode.
+- Dynamic output hotplug handling beyond first output.
+
+## Decisions
+
+### D1: Output mode storage
+- Store `output_width`, `output_height` from `wl_output.mode`.
+
+### D2: Default size computation
+- After roundtrip, if output info available, set logical size; else fallback 1024x576.
+
+### D3: Fallback
+- 1024x576 logical (~80% of 1280x720) when no output info.
+
+## Open Questions
+
+None.