[codex] Define vertical slice pipeline

[RollNuts]

Purpose

Adds the focused vertical-slice production pipeline for #5.

Closes #5 when later implemented; this PR defines the engine/tool flow and first-week validation path.

Scope

• docs only
• code
• asset
• build/release
• security/legal

What changed

• Adds docs/VERTICAL_SLICE_PIPELINE.md.
• Sets Godot 4.7 as first implementation candidate, while keeping Unity/Unreal as comparison paths.
• Defines Maya/Blender -> PixelOver -> Aseprite -> Godot flow.
• Defines first-week milestones and cutover criteria.
• Includes commercial-safety notes for free assets.

Public Safety

• No secrets or private data.
• No paid/API output.
• No third-party media committed.

veripsa core

Please include this PR in merge-order analysis with #8/#9/#10/#12. This likely depends on asset policy #12 and direction #8, and should precede actual engine project scaffolding.

Validation

• git diff --check passed in the worktree.

[RollNuts]

@veripsa core include this in the merge-order set: #8 #9 #10 #11 #12. This vertical-slice pipeline should likely wait for asset policy #12 and direction/protagonist specs before actual engine scaffolding begins.