docs: record editor-first UI-scene convention (ADR 0001)

Capture the direction change from M3-b: UI screens are authored editor-first
as .tscn node trees (previewable/arrangeable in the Godot editor), scripts do
on-load work only — never build the tree in _ready(). Add ADR 0001, a
"UI scene conventions" how-to in client/docs, a §16 mandate in the charter,
and mark M3-a/M3-b done in the roadmap.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/adr/README.md

@@ -4,7 +4,7 @@ One file per decision: `NNNN-short-title.md`. Record the context, the decision,
 
 ## Index
 
-- _none yet_
+- [0001 — UI screens are editor-authored scenes, not code-built](0001-editor-first-ui-scenes.md) (accepted)
 
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