fix: isolate .NET 11 Razor support in preview builds

[vicancy]

Related to #11171.

Add opt-in net11.0 DocFX assets for the .NET 11 RC1 Razor source generator while preserving stable compiler dependencies in ordinary builds. Measure every nightly tool target so the compatibility report exposes both the fix and the older tools' remaining limitations.

• Normal builds target net8.0/net9.0/net10.0 with Roslyn 5.6/5.6/5.9. A single RoslynVersion property selects the version per target for all eight compiler packages. DOCFX_PREVIEW_BUILD=true additionally builds net11.0 with Roslyn 5.11.0-1.26425.128; the public preview restore source and experimental symbol API adaptations apply only to net11.0.
• Nightly builds and tests all four targets. Each nightly net8.0/net9.0/net10.0/net11.0 tool runs all five SDK/project pairs and both Basic/Razor scenarios: 40 nightly cases plus 10 released net10.0 controls, 50 total. PR smoke remains net10.0.
• Installations, work directories, and logs are isolated by tool target. Reports distinguish requested and observed tool targets, reject incomplete matrices and fallback runtimes presented as measured results, and display the target in page headers and CI summaries. Earlier reports remain readable with their original coverage.

Tool target and project target are independent. SDK 11 Razor projects need the net11.0 preview tool even when the project targets net10.0. Older tools retain stable Roslyn and their incompatibilities remain visible. Direct consumers of the preview Docfx.Dotnet net11.0 library assets also need Microsoft's public dotnet-tools source.

Full test and 50-case matrix evidence at 1dd3822:

• PR CI passes on Windows, macOS, and Linux, including net10.0 packaged smoke. Format, coverage, and visual checks pass.
• Validation-only nightly passes four target-framework test suites, packaging, installed-tool checks, and reporting. Its 50-case report and logs record the clean commit, exact tool targets, and consistent package hashes. All cases were measured: 40 passed, 10 incompatible, no unavailable cases or infrastructure errors.

Package	Tool target	Passed	Incompatible
Latest stable	net10.0	8	2
Nightly	net8.0	7	3
Nightly	net9.0	7	3
Nightly	net10.0	8	2
Nightly	net11.0	10	0

The net8.0/net9.0 tools are incompatible with SDK 10 and SDK 11 Razor; the net10.0 tools are incompatible with SDK 11 Razor. SDK 11 is tested with both net10.0 and net11.0 projects. These observations remain visible and do not block reporting mode.

All 55 reporting tests pass, including 50 distinct case identities, missing-target and fallback detection, package identity, rendering, and archive export. A local 24-case comparison also completes without environment errors. Inspection of the actual package confirms stable Roslyn for net8.0/net9.0/net10.0 and preview Roslyn only for net11.0.

Current-head verification (f8eb636c2): complete normal and preview solution restores pass. The resolved dependency graphs for all four targets match the previous configuration exactly, including the net8.0/net9.0 exact version pins; ordinary restore uses no preview feed.