V3: render isolated groups in editor and runtime pipelines

[git-chad]

The renderer previously processed every layer in one global chain, so it could not give a group its own effect scope. Internal renderer frames now accept isolated groups: each group renders its children against transparent targets and blends the combined result into its parent. A threshold effect inside a photographic group affects that group while the surrounding background and upper layers retain their colors.

Stacked on #153; base branch: git-chad/v3-alpha-compositing. Part of the V3 integration in #150. Nothing in this stack merges to main until the required V3 roadmap is complete.

• Support nested groups, group opacity/blend mode/visibility, and top-first ordering at every depth in editor preview/export and runtime canvas/headless paths.
• Preserve child pass instances during reparenting, propagate animation/export preparation through visible groups, resize group targets, and dispose removed group resources.
• Reject duplicate IDs and nesting beyond eight levels before mutating live passes. Ignore obsolete compile callbacks after a pass is removed or replaced.
• Keep existing flat projects and shader configs unchanged. Editor grouping controls, saved groups, undo, public shader-config group export, and coverage masks follow in the next slices; this does not complete roadmap step 1.3.

Validation: bun run test:composition passes all frozen PNG/project/blend baselines, the existing 34 media and 100 alpha samples, and 52 new group checks. The group suite uses real editor/runtime pipelines and verifies scope, overlap, ordering, nesting, visibility, reparenting, resize/disposal, transparent external input, the headless entry point, and exact preview/PNG parity. Nested image orientation and soft edges match the flat render. bun run check, runtime and MCP typechecks, and changeset validation pass; only the pre-existing local lint warnings remain. No frozen baselines were changed.

Each group currently owns two full-resolution RGBA16F targets (about 32 MiB at 1080p), reused between frames. Large-scene performance and target pooling remain follow-up work. No new UI controls are exposed in this renderer-only slice.

[vercel]

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Project	Deployment	Actions	Updated
shader-lab	Ready	Preview	Oct 1, 2026 7:05am UTC

[greptile-apps]

Confidence Score: 4/5

The PR should not merge until stale media callbacks are isolated from replacement passes; the nested export reporting issue should also be corrected.

Summary

This PR adds isolated, nested composition groups to the editor and runtime rendering pipelines.

• Introduces composition-tree validation, recursive paint ordering, and per-group transparent render targets.
• Preserves passes across reparenting and propagates resizing, animation, export preparation, and disposal.
• Adds editor/runtime group coverage for isolation, ordering, opacity, visibility, nesting, lifecycle, and export parity.
• Two lifecycle and error-reporting issues remain in the editor pipeline.

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