Add Write-xJobProgress: mirror background job progress

[exactmike]

Summary

• Adds Write-xJobProgress, graduating the WIP/JobProgress.ps1 stub into a real exported function that mirrors Write-Progress calls made inside a Start-Job/Invoke-Command -AsJob scriptblock into the caller's session.
• Tracks the latest progress record per distinct ActivityId across all ChildJobs (not just ChildJobs[0]), preserving ParentActivityId nesting, and falls back to the job's own .Progress for job types with no child jobs.
• Assigns stable, collision-free Write-Progress -Id values by drawing from the same counter New-xProgress already uses, keyed internally by Job.InstanceId (not the recyclable .Id) so a reused job id can never inherit a stale mapping.
• Auto-completes and retires mirrored bars once a job leaves Running, and never reprocesses a retired job's leftover progress records on a later call.
• v1 is intentionally a lightweight write-only passthrough — no $script:ProgressTracker registration, no throttling/timer semantics. A -xParentIdentity parameter to nest job progress under a caller's own xProgress instance is flagged as a fast-follow in AGENTS.md/README (the internal id map is already independent of the tracker, so this is additive later).
• Updates xProgress.psd1 (exports, version bump to 1.1.0, release notes), README.md, en-us/about_xProgress.help.txt, and AGENTS.md to document the new function and retire the stale WIP note.

Test plan

• Invoke-ScriptAnalyzer -Path *.psm1 -Recurse — no new findings (only the 3 pre-existing ShouldProcess warnings)
• Invoke-Pester -Path ./Tests -Output Detailed — 134/134 passing, including 8 new tests in Tests/Write-xJobProgress.Tests.ps1 using real Start-Job/Wait-Job (parameter shape, single activity, stable id across calls, concurrent/nested activities, zero-ChildJobs fallback, empty progress, completion cleanup, id-collision isolation from live xProgress instances)
• Manual smoke test: two concurrent background jobs polled to completion via Write-xJobProgress, no exceptions, both jobs completed cleanly