feat(mobilecore): add in-process facade + net10.0-android target for mobile hosts

[IoannisMaras]

What

• Multi-targets officecli for net10.0 (existing CLI) and net10.0-android (new: library only, no Program.cs/CLI entry point) via <TargetFrameworks> + per-target OutputType/publish conditions.
• Adds officecli.mobilecore, a small in-process facade (MobileDocumentSession) over the existing document engine for Android/iOS hosts that need to open, edit, and render OOXML documents in-process: no subprocess, no MCP stdio, no named pipes, no shell.

Why

Embedding officecli directly in a mobile app process (so an on-device AI agent can edit a document without shelling out or running a local server) needs officecli to build as a library for net10.0-android, and needs a narrow, mobile-safe surface instead of the full CLI/MCP/plugin/installer surface. officecli.mobilecore intentionally exposes only: open/create, render-to-HTML, outline/get/query, set/add/remove, save. No CLI parsing, no watch server, no browser automation, no raw package writes, no arbitrary output paths.

Validation

The net10.0-android target has been building and running this exact flow inside a real Android app throughout development (WebView-rendered live preview, structured mutations end to end). Reproducible without Android, same facade, net10.0 desktop target:

$ dotnet run -c Release
MobileDocumentSession.Create("mobilecore-smoke-f9abc90a6b8d43288a3024ebfd320d3a.docx")
  -> created blank .docx, session open
Execute(Add paragraph) -> success=True message=/body/p[@paraId=00100000]
Outline() -> {"fileName":"mobilecore-smoke-f9abc90a6b8d43288a3024ebfd320d3a.docx","paragraphs":1,"tables":0,"images":0,"equations":0,...
RenderHtml() -> 50084 chars, contains added text: True
Save() -> ok
File on disk: True, 5193 bytes
Smoke test passed.

(3-line Program.cs + a ProjectReference to officecli.mobilecore.csproj. Happy to add this as a real test project in the repo if useful.)

[IoannisMaras]

Pushed a fix (87cf912) after testing this against a clean checkout more carefully: the original version made net10.0-android an unconditional second TargetFramework, which would have broken this repo's own release CI. dotnet publish src/officecli/officecli.csproj -c Release -r <rid> -o publish --nologo (the exact command build.yml runs, no -f) fails once a project targets multiple frameworks without one specified, on every OS in the matrix, and plain dotnet build/dotnet run for the desktop CLI would also start requiring the Android SDK/workload to be installed even for contributors who only touch the CLI.

Fixed by gating the Android target behind an opt-in property (IncludeAndroidTarget, default off). With no extra property, dotnet build/run/publish behave exactly as they did before this target existed - verified by running this repo's exact CI publish command and a plain dotnet run against the fix, both succeed with zero Android tooling involved. officecli.mobilecore (which always needs the Android target) requests it explicitly via ProjectReference Properties, so nothing downstream needs to remember to pass anything.

Should not affect anything else in this PR - same two commits, same scope, just a safer default on the new target.