feat: add Visual Parity Oven

[alowpoly]

Summary

• add the declarative Visual Parity Oven and its strict burnlist-visual-parity-data@1 contract
• expose validated, read-only Visual Parity data and dashboard routing
• reuse the Differential Testing dashboard shell for progress, delta charts, and sampled reference/candidate/diff cards
• cover payload validation, rendering, scenario routing, and server behavior

Why

Visual parity runs need a normalized, non-executable Oven surface that can display exact per-frame image differences without moving screenshot capture or comparison logic into Burnlist.

Impact

Projects can publish atomically prepared Visual Parity evidence through --oven-data visual-parity=<path>. Burnlist validates the payload and renders up to ten screenshot triplets sampled every 100 frames while retaining per-frame delta metrics.

Stack

This PR is stacked on #5 because it reuses the shared dashboard/live-update work introduced by the Streaming Diff Oven branch. Merge or rebase #5 first, then retarget this PR to main.

Validation

• node --test skills/burnlist/scripts/visual-parity.test.mjs (6 passed)
• npm run build:dashboard
• npm run verify (181 passed)