Add build-config and halo logging to DemDsmCreator

[vellusorvari]

What & why

Adds runtime observability to the DEM build / cross-source seam path — the context we kept having to infer by hand while diagnosing seams.

Per-source build summary (one line at the start of each source build):

Building source Q5232F1 (3000 m): 9 output tile(s) of 1000 m, 1 triangulation block(s) of 3000 m (whole-block), SeamMode TwoPass.

Surfaces output/block/source sizes, whole-block vs submesh, and the seam mode at a glance.

Halo trace on the cross-source seam path:

• when a source writes its own edge-frame (point count + file),
• when a missing neighbour band is extracted on demand under TwoPass,
• how many points each neighbour halo contributes when consumed, and a per-source neighbour total,
• debug lines when a neighbour is skipped (no band / no source .laz).

Example:

Halo [TwoPass]: wrote 191351 ground points for Q5232F1 -> Q5232F1_Halo_v0.0.0.0.laz
Halo [TwoPass]: extracted missing band for neighbour Q5232E2 on demand -> Q5232E2_Halo_v0.0.0.0.laz
Halo: Q5232F1 consumed 195280 points from neighbour Q5232E2.
Halo: Q5232F1 consumed halos from 5 neighbour(s).

Info level for the headline events, Debug for the per-neighbour skips. No behaviour change. Engine DLLs re-vendored for the Unity package.