Fix periodic coverage-map freeze on large fields (#34)

[chriskinal]

Fixes #34 — the web UI froze for ~1s and jumped the tractor forward periodically once sections were enabled, worse the larger the field. Device-verified on the internal simulator (no-boundary + boundary paths) through multiple bounds expansions.

Root cause

Coverage streaming is normally push-based (the host records every cell it paints and streams incremental deltas — cheap). The freeze came entirely from the full-state rebuild, which fired on grid changes and dumbly scanned the whole field.

Fixes (all in the coverage streaming path)

• Snapshot is now O(painted), not O(field). GetPaintedDisplayCells walks only the tracked painted bounding box of the display buffer (measured ~130ms → ~9ms on a 40ha field with 0.63ha worked), and it builds off the 10Hz broadcast loop so pose/tick frames never stall.
• Client drain is time-sliced (COV_DRAIN_BUDGET/frame) so a one-time seed paints over a few frames instead of blocking one.
• Bounds expansion resends nothing. The added ground is empty, so the host announces the new grid (reset=false) and the client re-anchors its existing coverage into the larger grid via an exact integer-offset pixel copy. A new CoverageInit.reset flag separates grow (re-anchor) from new-field/reload/cell-size-change (tear down + rebuild).
• Consistent snapshot read. The first no-boundary expansion crosses the 0.1→0.2m resolution band (rebuild path); the lock-free read could catch the display buffer cleared-but-not-yet-resampled and drop pre-expansion coverage permanently. Now materialized under _coverageLock.
• Edge-ribbon self-heal (pre-existing bug surfaced by the no-boundary test path): auto-init ClearEdges wiped an active section's ribbon while it stayed on, so the crisp worked-area perimeter silently never rendered. AccumulateEdge now re-opens a ribbon on a miss. Logically separable from the freeze fix — flag if you'd prefer it split out.

Testing

• Full solution test suite green (Models 146 / ViewModels 222 / Services 1026).
• Desktop head builds.
• On-device: no freeze/jump through repeated expansions; coverage stays pixel-aligned across each re-anchor; crisp edge renders from the start and survives expansions.

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