Turn the gauge hands with Core Animation, not SwiftUI (#69)

[evandhoffman]

Fixes #69.

The cause

The needle's travel time is the sampling interval, so it is always mid-sweep. As an animatable SwiftUI Shape, that kept the whole panel re-evaluating on the main thread at the display's refresh rate. A 10-second sample of a fresh 1.9.0 showed the main thread never idle, running display-cycle flushes and window layout back to back.

The change

• DialHand: the needle and peak mark are CAShapeLayers rotated by a CABasicAnimation. The render server runs the sweep, so the main thread wakes once per sample.
• An interrupted sweep continues from the angle the hand has reached. That angle is tracked in the view, because reading it back from the layer folds a 240° dial into ±180°.
• The hand is not hit-testable, so a tile's drag and right-click still land on the tile.
• Copy Image: ImageRenderer cannot draw a layer, so a copied dial uses the still NeedleShape / PeakMarkShape (liveHands: false).
• AGENTS.md: one Key Concepts bullet with the rule and the reason.

Measurements

Your saved layout (10 gauges, 13 charts, 1 s sampling), release builds via Scripts/make-app.sh. Each figure is the mean of six 5-second top readings after a 20-second warm-up.

build	CPU
main (c887960)	41.6%
hands not animated at all	8.6%
chart plot-width GeometryReader removed	40.6% (not the cause)
this change	6.8%

Tests

312 pass, and swiftformat --lint is clean.

• The mechanism. The sweep is a layer animation. An interrupted sweep starts where the hand is. The same reading twice starts nothing. Off screen, the hand snaps to its reading. A click passes through the hand.
• Copy Image. A copied dial has needle-coloured pixels. The live hands have none under ImageRenderer, which is why the still shapes exist.
• Geometry. The live needle and the still one land within 3 px of each other at readings 0, 0.25, 0.5, 0.9 and 1.0.
• The regression guard. The test fails on any SwiftUI .animation( or withAnimation in MonitorUI outside an allowlist (ReorderDrag's 0.12 s drop indicator). I checked it by putting the old animation back: it failed.

Not verified

• Nobody has looked at it on screen. The geometry test compares offscreen renders instead. Please check the dials by eye before merging (Scripts/make-app.sh && open .build/monitor.app).
• Mach ports. They sit at 330–340 in every build, including this one, over a 1.5-minute run. 1.9.0 held 450 after 6 hours, so a slow leak isn't ruled out. This change doesn't touch it.

[evandhoffman]

Checked on screen by the operator, 2026-09-25: the needles sweep smoothly and look the same as before.

--opus