Get-VisioPage: -ID is a real page ID, new -Index takes positions (#232)

[saveenr]

Summary

Fixes #232, in the direction agreed there: Get-VisioPage -ID becomes a real page ID lookup, and a new -Index parameter takes positions, so the two meanings are explicit.

• -ID now uses Pages.ItemFromID, the way Get-VisioShape -ID does for shapes. The docs, the cmdlet's own comment and the PageID column of Get-VisioPageCells already meant a real ID. Before, a real Page.ID gave the wrong page or threw: -ID 0 threw although the first page of a new document has ID 0.
• -Index <int[]> (new, its own pagebyindex set) takes 1-based positions, matching Visio's page numbering. These are exactly the numbers the old -ID accepted, so a script that used -ID as a position only needs to change the parameter name. (The other cmdlets' -Index parameters are zero-based, but they index rows in a shape's data, not pages.)
• -ID, -Index, -Name and -ActivePage cannot be combined.

Behavior change

Anyone relying on -ID as a position (including the old docs example Get-VisioPage -ID 1,3) gets different pages. The module changelog says so plainly and tells them to switch to -Index. A minor version bump for the module is appropriate.

Testing

• 9 new tests in PageCmdletBindingTests; 6 of them failed against the old code. They cover real-ID lookup (including ID 0 and several IDs, and IDs that differ from positions), -Index positions, out-of-range values failing in the lookup rather than in binding, and the parameter-set clashes.
• Debug build, local Visio: VTest.PowerShell 82/82, VTest.Models 104/104, VTest.Scripting 43/43.

Also

• VisioPowerShell/CHANGELOG.md [Unreleased]: Added and Changed entries.
• docs/TESTING.md, CLAUDE.md and the Models docs backlog entry (the Visio PowerShell docs note is release-gated) are updated.
• The companion docs change is in saveenr/VisioPowerShellDocs (base branch visiops_v4_docs).

Closes #232

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