feat(pp-page-generative): redesign template for multipage GenPage authoring

[TomProkop]

Rewrites the pp-page-generative template so it scaffolds a first-class GenPage project peer to pp-script-library, pp-plugin, and pp-pcf.

What changed

• Project file uses <ProjectType>GenPage</ProjectType> and references TALXIS.DevKit.Build.Dataverse.GenPage SDK.
• No IDs in csproj — solution-owned metadata under <solution>/uxagentprojects/<projectId>/uxagentproject.xml keeps stable identity across renames.
• Multipage by convention: any *.tsx at project root (excluding underscore-prefixed) is a page.
• Pages are discovered, compiled via Rollup, validated, and projected into the consuming Solution project automatically.

Companion PR

Build SDK: TALXIS/tools-devkit-build#64.

Status

Draft. Verified end-to-end against a demo project with two pages (Dashboard.tsx, LowStock.tsx) — pack and unpack round-trip cleanly.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com