[SOLID] [M8] Replace hardcoded generator registry with comptime GeneratorDescriptor array (covers M9)

[MichaelFisher1997]

Part of #776 — SOLID architecture debt umbrella.
Phase: P2 · Finding: M8

Problem

modules/world-worldgen/src/registry.zig:24-49 hardcodes all 4 generators in a static GENERATORS array. Adding worldgen-foo requires edits to 6 files (build.zig ×2, registry.zig ×2, root.zig, app.zig). Separately (M9), registry.zig:51-65 redefines a factory signature different from the GeneratorDescriptor.create the API already provides, forcing 4 trivially-delegating wrappers.

Evidence

registry.zig:24-49:

pub const GENERATORS = [_]GeneratorType{
  .{ .id = overworld.descriptor.id, ... .initFn = initOverworld },
  .{ .id = flat_world.descriptor.id, ... .initFn = initFlatWorld },
  ...
};

registry.zig:51-65 — 4 copies of the same delegation:

fn initOverworld(seed, gpa) RegistryError!Generator {
  return overworld.descriptor.create(.{.seed=seed,.allocator=gpa}) catch ...;
}

Fix

Replace GeneratorType/initFn with a comptime array:

pub const DESCRIPTORS = [_]*const GeneratorDescriptor{
  &overworld.descriptor,
  &flat_world.descriptor,
  &shadow_test_world.descriptor,
  &overworld_v2.descriptor,
};

Consumers call DESCRIPTORS[i].create(.{.seed, .allocator}) directly. Deletes the 4 wrappers and unifies the factory signature.

Verification

• nix develop --command zig build test (includes shader validation)
• If rendering-touching: capture headless screenshot baseline before/after (-Dskip-present)
• If behavior-preserving refactor: golden-output test must stay green

Constraints

• Small reviewable PR targeting dev; conventional commit (refactor: / feat:)
• Preserve current behavior unless this issue explicitly changes it
• Keep worker-thread RHI isolation intact (no RHI calls off main thread)

Notes

Resolves M8 + M9 together (they're one logical change). Runtime (world-runtime/src/world.zig:279) already calls registry.createGenerator(index, seed, allocator) — that API can stay, just re-implemented against DESCRIPTORS.

Tracking: #776

[github-actions]

Posted analysis comment to #792.

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