Update web port plan to support dual WebGL2 + WebGPU renderer

[ConstCorrectness]

Motivation

• The plan needed to support both WebGL2 and WebGPU targets instead of WebGL2-only to match the requested dual-port strategy.
• Provide an actionable migration approach so client runtime, shader pipeline, and assets can target both backends with minimal duplication.
• Clarify repository layout and tooling so a solo developer can begin implementing browser host entrypoints and CI for both graphics paths.

Description

• Replace the previous WebGL2-only rendering section with a dual-backend migration plan that defines a shared renderer API, backend scopes, feature rollout order, runtime selection/fallback, shader/asset pipeline recommendations, and backend-specific performance guidance.
• Update the recommended repository layout to include renderer/core, renderer/webgl2, and renderer/webgpu folders under the client workspace and mark the client as a TS host + dual renderer entrypoint.
• Add graphics tooling guidance for both WGSL (@webgpu/types, WGSL lint) and GLSL (glslangValidator) and recommend maintaining backend entrypoints plus runtime switching in the Vite build setup.
• Minor reflow of tooling/section numbering to accommodate the new graphics tooling subsection and corresponding documentation edits in docs/web_port_plan.md.

Testing

• Verified the new keywords and sections with rg -n "WebGL2|WebGPU|Rendering Migration|browser game shell|Vite|renderer" docs/web_port_plan.md and confirmed matches were present (success).
• Reviewed the produced diff using git diff -- docs/web_port_plan.md | sed -n '1,220p' to confirm the intended replacements and insertions (success).
• Printed the updated document fragments with nl -ba docs/web_port_plan.md | sed -n '1,140p' and nl -ba docs/web_port_plan.md | sed -n '168,290p' to validate layout, renderer subtree, and tooling sections (success).

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