docs: VisualBuilderPlan.md — visual builder GUI roadmap (synthesised from 9 parallel research agents)

Detailed component+UI+API+mapping plan for the visual model-builder GUI.
Synthesised from 9 parallel research agents:

3 code-extract agents:
  - cppmega.mlx: 22 BLOCK_BUILDERS bricks + 5 loss kinds + 4 optimizers
    + 3 rewriters + 12 presets + 8 device kinds + 9 parallelism kinds
    + 15 known footguns + 22+ named-dim ShapeExpr surface
  - ../cppmega: Megatron EP=4/8 production feature catalogue
  - ../nanochat: ~280 GPTConfig fields + memory_estimator.py (1265 lines
    reference) + 60 CUDA-only features

6 web-research agents (Tavily blocked on exhausted keys; reported clean
failure rather than fabricating):
  - Perplexity deep_research — WASM-native node-graph editor frameworks
  - Perplexity Pro (gemini-3.1-pro) — production visual ML builders
    market scan + Rust+WASM contenders
  - Brave + Exa fallback — top frameworks by stars/activity comparison
  - Exa neural + content — research papers + production tools survey
  - Perplexity reasoning (gemini-3.1-pro) — architecture pick analysis
    + JSON-RPC API contract design

Verdict: anywidget + React Flow 12 + ELK.js + Rust/WASM core +
JupyterLite/Pyodide for shareable demos. Ships dual-mode: real-PyTorch
in notebook OR Pyodide-symbolic in static URL. Pure-WASM (Rust+egui)
rejected for v1 — UX immature, no HTML overlays in nodes.

7-week implementation order: F-A JSON-RPC contract → F-B canvas+nodes
→ F-C sidebar → F-D anywidget shim → F-E JupyterLite static. Phase 2
(optional 3-6 months later): Rust shape engine WASM hot path, CRDT
multi-user, custom-brick plugin SDK.

Lives at repo root alongside Auto-FusionLayerBricks.md, VisualBuilderSpec.md,
ModelBuildSpec.md, ParallelismSpec.md.

Author: apstenku123