Fix stale hover latch on drag release, tooltip anchor, border off-by-one; pin gutter golden test, drop DPI framing

src/core/instrument/CLAUDE.md

@@ -242,3 +242,8 @@ slider couldn't. Two pure modules split the forward (draw) and inverse (edit) ma
   Channel-mode (D-E) bus-renegotiation design and the earlier Preserve-onset-latency
   framing in the S16 guardrails. Root `CLAUDE.md` is the current source of truth
   for both — do not reintroduce either superseded design.
+- **`keyboard_strip`'s width-uniformity guarantee is client-pixel only.** Its test sweep
+  covers client-pixel widths (including multiples standing in for larger client areas);
+  nothing in the instrument implements `IPlugViewContentScaleSupport`, so host-side DPI
+  scaling of the plugin window — which would resample the uniform integer key widths at the
+  physical-pixel level — is unverified.