Model structural surfaces and free-air elevation

[zoeyrose]

Important

This issue is implemented in the fresh MIT-licensed Go server under the replacement program. Its gameplay and content-design decisions remain authoritative. C, CPython, classic packet, file-path, and enum details in the preserved specification are historical evidence only; do not copy, translate, or structurally port GPL implementation code.

Replacement implementation contract

Make stable SurfaceID, support height, collision, lighting, and free-air elevation foundational Go world concepts. Generated protocol contracts and the shared Rust renderer/editor model consume them directly; there is no interim legacy sub-layer representation.

The server remains authoritative, consumes versioned compiled content, and exposes bounded generated Game Protocol 1 messages. Pure rules may use a specifically approved typed CEL environment. Starlark is not part of this issue unless the separate residual-scripting decision explicitly approves it.

Required verification

• Preserve every observable rule, balance decision, disclosure boundary, and anti-exploit invariant from the specification below.
• Add deterministic Go unit/property tests and wrapper-managed scenario coverage at the appropriate integration boundary.
• Add bounded malformed-input and persistence-failure cases where this feature accepts content, network, or stored data.
• Add Go/Rust protocol conformance fixtures for every new cross-process field; the client must not reconstruct authoritative rules from prose.
• Demonstrate that implementation and tests contain no copied GPL source/test material and execute no runtime Python.

Preserved product/design specification and historical implementation notes

Parent and roadmap

• Parent tracker: Ordered world model: compile, simulate, stream, render, and edit stable scene records atrinik#156
• Roadmap: Roadmap: replace classic Atrinik with a fresh MIT Go and Rust stack atrinik#168
• Roadmap milestone: M5
• Prerequisites: Migrate authored order directly to structural ordered scenes content#12, Render a large scenery viewport from streamed geometry and a bounded live area client#19, atrinik/client#17, World imagery: publish deterministic products through the shared renderer content#4

Outcome

Complete Stage B by replacing the seven-slot elevation/light/damage model with explicit bounded structural surfaces and free-air elevation.

Scope

• Give each walkable/structural surface stable local identity, elevation, terrain/collision properties, and light samples.
• Make living, item, effect, projectile, movement, falling, damage, and animation state reference a surface or explicit free-air elevation.
• Migrate lighting and wire state to the bounded surfaces that actually exist.
• Update authored schema, content, editor, checkers, scripts/plugins, runtime save/load, fixtures, and consumers together.

Superseded path / cleanup target

Remove NUM_SUB_LAYERS, author-facing/runtime sub_layer, sub-layer mailboxes, and residual numeric layer semantics after no current consumer remains.

Acceptance criteria

• Movement, falling, collision, exits, lighting, damage placement, and linked physical depths operate on explicit surfaces.
• Authored and mutable state preserve stable surface identity across save/reload and delivery.
• No client/server/editor/checker/script consumer requires numeric sub-layers.
• The old seven-sublayer model is deleted without flattening linked physical maps.

Validation baseline

Before: ordered display is modernized but gameplay/elevation still depends on slots 0–6.

After: explicit structural surfaces own support, height, collision, light, and event placement.

Run movement/falling/collision/light/damage matrices, multi-storey fixtures, save/reload/snapshot tests, whole-content validation, server checks, and native builds.

This issue is a bounded child of atrinik/atrinik#156. The parent remains open until every sibling child is complete.