PHLEGM v8 shared Compiler UI - Checkpoint 39: seven-good trimmed ETA

Selection / rolling history:
- Record only successful builds (exit code zero, not cancelled).
- Preserve the most recent 7 successful durations in chronological order.
- Once 7 successes are present, sort a temporary copy and exclude exactly
  ONE shortest and ONE longest duration. Average ONLY the 5 remaining durations.
- The excluded samples are excluded from the estimate, NOT removed from history.
  The next success evicts the oldest sample, not the shortest/longest.
- Tied shortest/longest durations are each excluded ONCE, not every tie.
- Before 7 successes are available, average all available successes with no
  exclusions. Zero successful builds means Duration unknown.
- Failed and cancelled builds never add samples, clear or reset successful
  history, or alter the existing rolling average.

Compatibility and display:
- The existing settings file and history array names are unchanged.
- Previous settings holding up to 5 successes migrate without resetting the
  saved durations; two more good builds fill the new 7-entry window.
- LastGoodDuration is the existing progress mode identifier; the supplied
  basis is now the trimmed mean. The countdown ETA and visible percentage
  use the SAME immutable basis for the duration of the current build.
- Compiler UI displays "Duration (trimmed avg 5/7 of 7 good)" when full and
  "Duration (avg k/k of 7 good)" during initial warm-up.
- Tools/Compiler-UI.ps1 remains project-neutral/shared for Mindrealm and PHLEGM.
  When syncing shared tooling, keep the identical engine and project-specific
  manifests/data-driven Compiler-UI.json per project.

Data-driven policy (tools/Compiler-UI.json):
  progress.knownDuration.goodBuildHistoryCount = 7
  progress.knownDuration.discardShortestCount = 1
  progress.knownDuration.discardLongestCount = 1

Validation: sample counts are integers; removed counts cannot consume all N.
See tools/Compiler-ETA-Trimmed-Mean-39.Tests.ps1 for native PowerShell
standalone test scenarios (run using pwsh -File on any supported host).
