feat: consolidate GPU viewer and runtime 4.2

[kiletw]

Task

• Consolidates completed work from TASK-033 through TASK-047.
• Establishes the merge-ready v3 baseline against master.

Changed

• Added OpenTK GPU preview with explicit CPU fallback, renderer/performance status, display-channel controls, and Screen/PMA blend parity fixes.
• Added the pinned official Spine 4.2 C# Runtime adapter with JSON/binary deterministic coverage and bounded Physics replay.
• Expanded the layer-focused workspace with multi-layer actions, transform controls, slot visibility/opacity/attachment selection, and non-structural Undo/Redo.
• Added drag/drop, manual atlas recovery, first-frame refresh, Dark/Light themes, screenshot/PNG-sequence export, and configurable Preview FPS (default 30).
• Replaced the legacy-only root solution with a 20-project v3 solution.
• Removed the v2 .NET Framework/XNA Viewer, Players, packages, resources, and unused XNA Runtime helpers.
• Relocated 484 active historical Runtime source files into matching runtimes/SpineRuntime.V*/src projects; all 484 are Git R100 moves and their per-version SHA-256 manifests match before/after.
• Updated English/Traditional Chinese README files, runtime paths, task reports, ADRs, evidence, and validation scripts.

Preserved behavior

• Runtime adapters from Spine 2.1.08 through 4.2 remain composed and covered.
• Deterministic CPU render/export remains available when GPU preview is unavailable or disabled.
• Project saves remain non-destructive *.spineviewer.json sidecars; Spine source files are not overwritten.
• The removed v2 implementation remains recoverable from Git history and the pre-PR master state.

Validation

• Root solution Release build: 20 v3 projects, 0 warnings / 0 errors.
• All 13 relocated historical Runtime projects: 0 warnings / 0 errors; 484 source manifests matched before/after.
• WPF UI shell: 74 AutomationIds; floating inspector, slot attachment, slot visibility, Undo/Redo, and duplicate-layer checks passed; build 0 warnings / 0 errors.
• Application smoke: Viewer project, PNG preview, edit history, validation, source isolation, and Preview FPS contract passed.
• scripts/test-v3.ps1: inspect/render/PMA/4.0/4.2 passed; 12 historical Runtime deterministic checks passed.
• scripts/test-v42.ps1 -Offline: official 4.2.22 JSON/binary inspect and deterministic renders passed; build 0 warnings / 0 errors.
• Active references to the legacy project/SpineLibrary: 0. git diff --check: passed.

Runtime/upstream impact

• Spine 4.2 is pinned to official spine-runtimes commit b81e5a58ed38704aee4f866f0e0ac672623ce914.
• Historical source relocation changes paths only; Runtime .cs contents were not edited.
• Project-owned V41 renderer bridges add V42-compatible namespace/signature conditionals; existing V41 upstream source is unchanged.

Risks and gaps

• Screenshot and PNG-sequence export currently capture only the primary scene layer while the viewport can show multiple layers.
• Runtime 4.3 is not implemented.
• Structural Spine source authoring/save-back is not implemented; edits remain sidecar-based.
• Structural layer add/remove Undo/Redo reconstruction is deferred.
• The exact large 116421.json asset still benefits from a long-load acceptance pass on the target machine.
• Spine 4.2 Physics beyond 10 seconds uses a bounded 600-step approximation.
• Historical Runtime provenance remains less exact than the clean pinned 4.1/4.2 snapshots; TASK-047 proves relocation identity, not upstream origin.

Documentation updated

• README.md, README_zhTW.md
• docs/ai/00-context-index.md
• docs/ai/04-runtime-matrix.md
• docs/ai/07-migration-plan.md
• TASK-033 through TASK-047 and related ADR/evidence files