PHLEGM v8: Shared compiler UI next-build ETA checkpoint 72

The existing shared compiler engine already implements the requested data-driven
7-success trimmed mean (discard exactly one shortest and one longest, average
remaining five), preserving all seven successful samples in chronological order.

This checkpoint adds a visible next-build prediction before each build.
The existing running ETA counts down from the same immutable prediction in
mm:ss.fff, and is never recalculated mid-build.

No successful history: unknown duration.
1-6 successes: provisional arithmetic mean; no trimming.
7 successes: discard one shortest and one longest; mean remaining five.
8th success: evict oldest, retain newest seven, recompute trimmed estimate.
Failed/cancelled builds: preserve history, prediction and prior average.

Compiler-UI.ps1 is byte-identical in PHLEGM and Mindrealm distributions.
Policies are driven by each project's own Compiler-UI.json.

Validation: extracted browser JavaScript syntax passed Node 22 --check;
forecast UI scenarios and millisecond countdown passed Node sandbox tests.
PowerShell production tests and hands-on build have not been run here,
because PowerShell is not available in this environment.
