standalone: owner review fixes -- paged streaming in Core, no LOD fins, LOD0 stair patches fixed

From the owner's fly-through ("very impressive, in some cases slightly better than the original"):
- Streaming: Core compiles world-aligned fine-detail pages for tiles entering the view and releases
  them when they leave; the viewer keeps resident tiles. One chunk crossing: 144 new tiles, 1,822
  kept (was a full rebuild). A tile built as a late page hashes identically (density, mesh, payload,
  topology) to the same tile from an initial build.
- LOD>=1 fins: off-surface planar connector panels are now checked against the source surface and
  projected onto it; coarse audit 0 off-surface panels; far survival passes, bad-shell still fails.
- LOD0 stair patches: 2,630 sign-changing edges had one endpoint at the fixed +-4.125 fallback
  (carve surfaces were excluded from distance storage); distances now completed analytically, 462
  packed signs corrected; the audit now fails on any sign edge missing either endpoint (0 of 952,997).
- W1/W8 hashes equal; routes 5642/5642; look, passage and far audits pass.
Verified by me, interleaved (8 workers, cold): 1.014 s vs 1.008 s for aeeedaa, no regression.
Open: a chunk crossing still costs ~1.2 s of page compilation.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MmSoF64BbRFmk9C3p1VsBJ

Standalone/Core/API.md

@@ -48,6 +48,32 @@ build.includeFarWorld = true;
 if (!world.compile(build, hostScheduler, error)) { /* report error */ }
 ```
 
+For a streamed shape world, compile the initial camera region once, then add deterministic detail
+pages before generating any newly desired fine tile outside that region:
+
+```cpp
+const auto entering = world.desiredTiles(newCameraVoxels);
+VoxelForgeCore::TileGenerationOptions options;
+std::size_t pageCoveredTiles = 0;
+if (!world.compileTileRegion(entering, 8, hostScheduler, error, &pageCoveredTiles)) {
+    /* report error */
+}
+for (const auto& key : entering) {
+    if (auto mesh = world.generateTile(key, options, error)) { /* upload and retain */ }
+}
+world.retainTileRegions(entering);
+```
+
+The Core groups fine tiles into pages using absolute tile coordinates, so a page's contents do not
+depend on the camera or the order in which the player reaches it. Pages cover a 128-voxel cube (or
+one larger coarse tile), with an 8-voxel sampling apron. `compileTileRegion` skips LODs at or above
+`fullResolutionLodCount`, keys covered by the initial `ShapeBuildRequest`, and pages already
+resident. Its optional output counts requested fine tile keys that needed a newly compiled page;
+it does not count the page's internal lattice samples. After all pages needed by the next desired
+set compile, call `retainTileRegions` with that set to release pages no longer needed. The Core
+owns page lifetime and mesh data; the host keeps still-visible meshes resident until replacement
+tiles are generated and uploaded.
+
 ## Queries and tiles
 
 `densityAt(position)` returns signed density; negative is solid and nonnegative is air.