Collect code coverage in parallel runs

[nohwnd]

Summary

Run.Parallel previously fell back to a sequential run (with a warning) whenever CodeCoverage was enabled. This enables coverage collection during parallel runs instead.

• Each worker measures its own file with breakpoints and returns its per-location hits. The default profiler/tracer keeps state in a process-global static and is not concurrency-safe across worker runspaces, so breakpoint mode is forced for parallel + coverage.
• The parent merges every worker's hits, adds the coverage of any #pester:no-parallel files it runs in-session, and lets the existing Coverage plugin End step emit the single merged report and output file (JaCoCo/Cobertura) plus run.CodeCoverage.
• PowerShell 5.1 stays unsupported for parallel — it still falls back to a sequential run (collecting coverage sequentially) with the version warning, since it lacks ForEach-Object -Parallel.

Changes

• src/functions/Pester.Parallel.ps1 — workers enable breakpoint coverage, suppress their own report/file write via a CodeCoverageSkipReport module flag, and return a lightweight per-location hit projection.
• src/functions/Coverage.ps1 — new Merge-CoverageFromParallel and Convert-CommandCoverageToProjection helpers.
• src/functions/Coverage.Plugin.ps1 — End step honors the skip flag and reads UseBreakpoints from the captured plugin configuration.
• src/Main.ps1 — removes the coverage fallback/warning; collects worker coverage, measures #pester:no-parallel files in-session, merges everything, and hands it to the Coverage plugin for the single report.
• tst/Pester.RSpec.Parallel.ts.ps1 — replaces the obsolete "falls back with warning when CodeCoverage" test with two tests: parallel coverage equals sequential, and coverage from a #pester:no-parallel file is merged in.

Verification

• Parallel coverage matches a sequential run exactly (analyzed/executed/missed counts, percent, executed lines).
• Mixed #pester:no-parallel case merges correctly; JaCoCo + Cobertura XML valid; output file written; no stray worker output files.
• All 21 parallel P-tests pass (incl. 2 new coverage tests); all 12 coverage P-tests pass.
• No new PSScriptAnalyzer custom-rule violations in the changed lines.

Related to #1704.