backlog: centre provision for sub-threshold additions

Author: songololo

docs/backlog.md

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+# Backlog
+
+Open design questions and deferred work, roughly ordered. Items move out of here
+into releases once decided.
+
+## 1. Centre provision for sub-threshold additions to existing fabric
+
+Post-processing adds a mixed-use centre wherever new development lacks provision of
+its own, and the addition threshold is small (a centre must reach a handful of
+otherwise-unserved cells). A new patch grown against existing fabric can therefore
+earn its own commercial area even when its population is below the minimum-settlement
+threshold (default 1,000 people), the same threshold below which a *detached* patch
+would be pruned as non-viable.
+
+Question under debate: is that provision worth making? A sub-threshold addition could
+not stand as a settlement, and the adjacent existing centre demonstrably serves it
+(growth required a centre within the walk). The candidate rule: new development only
+earns its own centre once its population reaches the minimum-settlement threshold,
+unifying "viable as a settlement" and "warrants its own centre" into one number.
+
+Before deciding: count, across the seven scenario galleries, how many added centres
+serve sub-threshold catchments, and what served-coverage and centre-per-person
+figures look like with and without the unified threshold.