fix: share tool templates across target frameworks

[vicancy]

Summary

Restore the existing shared-template layout for the multi-target .NET tool. The release currently packs templates three times, once under each of tools/net8.0/any, tools/net9.0/any, and tools/net10.0/any.

• Collect templates from the real publish directory, remove the framework-specific package entries by normalized source path, and add one shared templates/ directory using the first target framework.
• Enable Docfx.DotnetToolMode while packing without incorrectly excluding the SDK's internal publish phase.
• Regenerate the runtime configuration when alternating pack and publish, including --no-build, so installed tools use shared templates and normal/self-contained publish outputs continue using adjacent templates.
• Add a real package regression script and wire it into the existing Windows/macOS/Linux CI jobs. It checks the archive layout and all three runtime configurations, installs the package for .NET 8/9/10 into isolated directories from a local-only feed, verifies every exported template against the built assets, and generates HTML and PDF. It also verifies a normal publish output after the pack/publish/no-build cycle.

No framework support, PDF functionality, source maps, dependencies, release tags, or signing behavior are removed or changed. This affects future packing, not already-published packages.

Root cause

The old PackagePath.StartsWith("tools/<tfm>/any/templates/") condition does not match SDK 10's split destination metadata (PackagePath = tools/<tfm>/any/, with the remaining path in RecursiveDir). Also, SDK 10 sets both _IsPacking and _IsPublishing during tool packing, so the old !_IsPublishing condition suppresses the shared-template runtime switch.

The fix does not depend on that destination-metadata shape. It identifies the actual template source files instead.

Measured package size

Same commit, generated templates, dependencies, version and local unsigned packing process:

	Before	After
Package bytes	94,713,551	74,252,897
Package MiB	90.33	70.81
Framework-specific template entries	1,350	0
Shared template files	0	450

Reduction: 21.6%. All 146 template source maps are retained once instead of three times. These are comparable local unsigned packages, not a claim that the final signed release will have exactly the same size.

Validation

• New checks fail on the baseline package: no shared templates, three duplicate directories, and missing runtime switches for all three frameworks.
• An additional publish -> pack --no-build check reproduced stale runtime configuration before the final fix and passes afterward.
• Windows, SDK 10.0.401: actual installed .NET 8 and .NET 10 tools pass version, template list/export, all 450 exported-file hash checks, HTML content/assets, and PDF generation with warnings treated as errors.
• Framework-dependent and actual self-contained win-x64 publish outputs pass the same template/HTML/PDF checks without tool-mode leakage.
• All three target frameworks build; local .NET 9 execution is unavailable because that runtime is not installed. The added CI test runs all three frameworks on each existing OS job.
• Existing docfx.Tests: 84 pass; 32 schema cases fail on an unchanged checkout-name assumption (PathHelper requires a directory named docfx or docfx.sln, while this isolated checkout has another name and the repo uses docfx.slnx). Two representative cases reproduce with the production project restored to unmodified main. This unrelated helper is not changed.
• PowerShell syntax, CI YAML parsing, project formatting, and diff checks passed. Dependency restores used the approved feeds.

CI package metrics

Each distribution-test step now writes a GitHub Actions job summary and console table containing the actual package frameworks, template directories/files, remaining framework-local entries, duplicate file copies avoided, compressed/expanded shared-template size, and actual package size. Estimated duplicate data avoided is explicitly labeled and excludes ZIP entry overhead; it is not presented as a separately built before/after measurement.