feat(walls): model deterministic visual-variant groups

[zoeyrose]

Parent: atrinik/classic#332

Outcome

Add an authored, validated content representation for opt-in deterministic visual-variant groups so render-equivalent walls can vary without encoding that choice as a different gameplay archetype.

The first fixture is the existing fwall_sand1a/fwall_sand1b pair. The schema must remain useful to later generated forms/materials and coordinate with #61's stable semantic visual-family catalog rather than deriving identity from paths or face-table order.

Scope

• Define stable visual-family/group IDs, canonical member IDs/order, optional integer weights and authored seed, fixed/automatic mode, and a required complete fallback face.
• Declare the shared wall form/compatibility identity needed to prove members are interchangeable for presentation.
• Extend the canonical content schema, parser/generator, documentation, and validation. Do not hide the contract in unknown free-form object properties even if Classic currently preserves them.
• Validate referenced face existence and compatibility across dimensions, alpha silhouette/anchor, draw_double, animation/turning, glow, visibility blocking, and gameplay-significant archetype fields.
• Emit deterministic runtime metadata for Classic without changing existing face IDs or deleting the legacy a/b archetypes.
• Coordinate names/ownership with the reusable recipe generator so one family can later name generated members without a parallel ID system.

Validation cases

• Valid two-member full-wall group using the sand faces.
• Stable order independent of source discovery or packaging order.
• Explicit fixed a and fixed b representations plus automatic selection.
• Missing face, duplicate member, zero/negative/overflowing weight, unknown version, and missing fallback.
• Same-size but different silhouette/anchor, different dimensions, animation, glow, draw_double, or gameplay semantics.

Acceptance criteria

• The authored format has a version, stable semantic identity, deterministic canonicalization, and generated documentation/schema artifacts where required.
• Reordering files or runtime face numbers does not reorder members or change selection test vectors.
• The sand group validates while every incompatible fixture fails with a field/member-specific diagnostic.
• Fixed selection is representable without inventing a duplicate gameplay archetype, while both existing sand archetypes remain valid fixed legacy identities.
• A missing/unknown group or unsupported version names an ordinary complete fallback and cannot produce a partial presentation.
• Generated Classic metadata is byte-identical across repeated clean builds and carries exact input identity/provenance.
• The schema explicitly remains opt-in; unregistered wall objects and exceptional families retain their current singular face/animation behavior.

Non-goals

• Editing production maps; that is the sand-pilot child.
• Generating new pixel art or extracting templates from flattened PNGs.
• Applying generic variation to arbitrary same-sized faces.
• Replacing the stable identity/catalog work already owned by #61.