docs(decisions): open Q-108 — subterranean/enclosed settlement morphology

The D-233/D-237 built-world fill model is implicitly a surface model
(roofed-coverage fraction, operations surface, scatter-vs-concentrate on
open ground), but bodies.settlement_pattern already carries enclosed
morphologies (underground_concentrated, cave, domed, underground_complex)
that nothing in the chain reads — not in the D-199 read-set, not in
city_context_reader, not in skeleton_gen. Engine z-level primitives exist
(D-110, D-106, UndergroundComplex) but no morphology switch connects them
at the built-world layer, so a body like Vuurkloof (GJ 35) would generate
as a surface town against its underground-concentrated lore.

Q-108 schedules the surface-vs-enclosed morphology debate before the
Phase 4 content pass authors built form for enclosed-habitat bodies.
Tracked by ticket #1018. Surfaced via Vuurkloof during the
system-economic-specialization workshop.

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

decisions/questions-architecture.md

@@ -411,4 +411,12 @@ Technical foundation questions: engine, protocols, data structures, performance,
 
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-*56 questions (11 resolved, 1 partially resolved, 44 open). Last updated: 2026-05-25.*
+### Q-108: Subterranean / domed / sealed-habitat settlement morphology — does the built-world fill model need a surface-vs-enclosed branch
+- **Status:** Open — raised 2026-05-31 (system-economic-specialization workshop follow-up, via Vuurkloof / GJ 35)
+- **Question:** The economic-built-world fill model (D-233, re-amended by [D-237](architecture.md#d-237)) is implicitly a **surface** model — it derives a roofed-coverage fraction, an operations-surface remainder, and a concentrate-vs-scatter spread across open ground. But `bodies.settlement_pattern` already carries non-surface morphologies for real bodies: `underground_concentrated` (e.g. Vuurkloof / GJ35c, built into ravine walls), `cave`, `domed`, `underground_complex` (~12+ bodies on `underground_concentrated` alone, more across the others). **Nothing in the current chain reads `settlement_pattern`** — it is not in the [D-199](architecture.md#d-199) read-set, not consumed by `city_context_reader`, not branched on in `skeleton_gen` / D-233 fill. The engine *primitives* exist ([D-110](architecture.md#d-110) signed `base_z`, the `UndergroundComplex` reservation, [D-106](architecture.md#d-106) vertical scale — "a deep mine is an inverted skyscraper"), but no morphology switch connects `settlement_pattern` to them at the built-world layer. So an enclosed-habitat body like Vuurkloof would currently generate as a *surface* geothermal town (conduits and exchange stations spread across open ground), contradicting authored lore (underground-concentrated, ravine-wall construction). Does the fill model need a first-class **surface-vs-enclosed (subterranean / domed / sealed-habitat) morphology branch**, and where does it live — (a) `settlement_pattern` as a hard-gate input to D-233/D-237 fill selecting an enclosed coverage/vocabulary model; (b) a separate morphology layer above D-233 the economic vocabulary plugs into; (c) treat domed/sealed (pressurized surface envelope) as distinct from true subterranean (excavated z-negative)? And how does verticality (D-106/D-110) compose with the coverage model when a settlement is primarily vertical/subsurface, and how does `morphology_zone` (D-228/D-234, terrain-driven street geometry) behave when there is no open street plane?
+- **Context:** D-237 *inherited* D-233's surface assumption; it did not introduce the gap and does not block on it (surface bodies — the majority — are correct today). But enclosed-habitat bodies are authored lore and will read wrong until resolved. Wants its own debate/workshop with Tyre (z-level architecture), Burnelli (coverage model), Miri (which bodies, what they must read as), Araminta (how enclosed interiors are visually distinct). Tracked by ticket #1018; schedule before the Phase 4 content pass authors built form for enclosed-habitat bodies.
+- **Cross-reference:** [D-233](architecture.md#d-233) (surface coverage model — would be amended), [D-237](architecture.md#d-237) (authored specialization layer — inherits the surface assumption), [D-199](architecture.md#d-199) (read-set — would need `settlement_pattern`), [D-220](architecture.md#d-220) (density / vertical pressure), [D-106](architecture.md#d-106) (vertical scale), [D-110](architecture.md#d-110) (signed z-levels), [D-228](architecture.md#d-228)/[D-234](architecture.md#d-234) (morphology zone / street geometry), [D-196](architecture.md#d-196) (SettlementClass — orthogonal; this is morphology, not active/ghost)
+
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+
+*57 questions (11 resolved, 1 partially resolved, 45 open). Last updated: 2026-05-31.*