Use analytic springs and instrument resize barriers

Share a stable damped oscillator solver across scalar and rectangle animations while preserving retarget and explicit no-crossing presentation policies. Add deterministic traces, scrolling and zoom continuity coverage, and pause-aware advancement.

Measure resize barrier and handoff timing behind resize-metrics, expose the feature through the release installer, and wait for the intended backend in isolated portal tests.

compositor/Cargo.toml

@@ -29,3 +29,7 @@ xcursor.workspace = true
 
 [dev-dependencies]
 tempfile.workspace = true
+
+[features]
+# Opt-in counters/histograms with a per-client exit summary.
+resize-metrics = []