docs: add E2EMatrix.md design — Playwright-driven GUI -> API -> train matrix

Forward-looking design for cppmega-mlx-pa3 (E2E Coverage Matrix epic),
drafted in the same style as VBSpec / MBSpec / PSpec / ContractProbe /
VisualBuilderPlan.

Doubles down on GUI-first: every test starts at / and walks through the
UI like a live user (preset launcher / tokenizer playground / data
inspector / run modal). Backend is the real FastAPI uvicorn instance;
canvas is the real React Flow tree; gradients are real mlx.

Mini-spec under 200M params hard cap (preferred <100M): H=128,
DEPTH=2, S=64, num_experts=4. 3B does not fit (~60 GB at 2k blocks);
1B is borderline. GPT-2 small (50257) is the largest vocab we use.

Matrix shape: 66 non-xfail Raschka archs × 4 tokenizer × 4 parquet
= 1056 smoke cells; 12 family-rep × 4 × 4 = 192 real-train cells
(2 grad steps each).

7 stages: E-0 wire App.tsx (pre-condition; preset launcher / run /
sharding accept / heartbeat / tabs are stubbed today), E-1 fixtures,
E-2 Playwright scaffolding, E-3 preset matrix, E-4 mini-train matrix,
E-5 specialised scenarios, E-6 CI + report.

Epic + 7 sub-tickets created in bd as cppmega-mlx-pa3 + .1..7 with
dependency chain (E-0 blocks E-1/E-2; E-1+E-2 block E-3; etc).

Author: apstenku123