Add opt-in contentless tileset root

[Xananthium]

Summary

• add an opt-in --no-root-content CLI switch (default remains false)
• pass no rootSourceObj to the existing tiling path when requested, producing its legal contentless ADD root and no root.b3dm
• document the renderer compatibility tradeoff

Motivation

Privacy-audited and geometry-decimation workflows may intentionally publish only separately accepted child tiles. The automatically generated whole-model root is then a redundant artifact with a distinct release/audit surface. This switch lets those workflows omit it without post-processing tileset.json or deleting output after generation.

The option does not crop or validate geometry. It only selects Obj2Tiles' already-supported contentless-root behavior. It is deliberately off by default because some renderers do not descend into children of a contentless root.

Tests

• dotnet test Obj2Tiles.Test/Obj2Tiles.Test.csproj -c Release: 82 passed
• dotnet test Obj2Tiles.Library.Test/Obj2Tiles.Library.Test.csproj -c Release: 135 passed, 1 explicit test skipped
• CLI smoke test with --lods 1 --divisions 0 --local --no-root-content: exit 0, no root.b3dm, root content: null, root refine: ADD, child B3DM present

[Copilot]

Pull request overview

Adds an opt-in --no-root-content CLI switch to omit generating root.b3dm, relying on the existing tiling behavior that produces a legal contentless tileset root (with refine: "ADD" and content: null). This supports workflows that only publish separately reviewed/audited child tiles and want to avoid emitting a redundant whole-model root artifact.

Changes:

• Document --no-root-content in the README options table.
• Gate rootSourceObj selection/compression in Program.cs so the tiling stage receives null when --no-root-content is set.
• Add CLI option parsing tests for NoRootContent.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
README.md	Documents the new --no-root-content switch and its renderer compatibility tradeoff.
Obj2Tiles/Program.cs	Skips root content generation path when NoRootContent is enabled, passing rootSourceObj: null to tiling.
Obj2Tiles/Options.cs	Introduces the --no-root-content option on Options.
Obj2Tiles.Test/OptionsTests.cs	Adds tests ensuring the new flag defaults to false and parses correctly.