whiteboard: keep the highlighter translucent while drawing, and give it a chisel nib

[haraldschilly]

While drawing with the highlighter the stroke looked opaque, and only turned translucent once the mouse button was released. Cause: the live preview was drawn incrementally — each mousemove stroked the last two points onto the preview canvas, so the overlapping round ends of consecutive segments composited on top of each other and built up to full opacity.

What changed

• Preview redraws the whole path in one operation, coalesced through requestAnimationFrame. drawCurve now always paints a curve with a single stroke()/fill(), so a translucent color never composites with itself.
• Point thinning: preview points closer than 1px to the previous one are dropped, so a high-frequency pointer doesn't blow up the path. (The existing thinning when the element is committed is unchanged.)
• Chisel nib for the highlighter (nib: "chisel"): an upright rectangle 2*radius tall and a fraction of that wide, swept along the path — a wide band when swept horizontally, a thin line when moved vertically, like a real highlighter. Elements without nib keep the old round rendering, so existing drawings are unaffected.

Two related bugs found while verifying:

• Pen cleared its canvas after translating by pad, leaving a pad-wide strip along the top/left uncleared. That strip holds the top half of a wide stroke, so every re-render of a translucent stroke composited it onto itself and it got darker over time (measured 0.4 → 0.64 → 0.78 as more elements were added).
• The preview canvas had no explicit CSS size, so it was laid out at its bitmap size; the old code compensated by drawing at 1/devicePixelRatio. It is now sized in CSS pixels and drawn at full device resolution, so the preview also lines up with the finished element on HiDPI displays.

Verification

Headless Chrome against local Lite, sampling canvas pixels during the drag and after release, at devicePixelRatio 1 and 2:

	during drag	after release
before	maxAlpha 255 (avg 244)	102
after	maxAlpha 102	102

Preview and finished element also agree on position/size to within 0.5px at both DPRs. Frame timings stay at 16.7ms median/p95 for a 2000-point stroke. Added elements/pen.test.ts covering the single-composite property and the chisel geometry.

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[blacksmith-sh]

Found 10 test failures on Blacksmith runners:

Failures

Test	View Logs
account/tests/membership-package-manager.test.tsx/ lets admins add site-license delegates with admin user search	View Logs
membership package managers/ lets admins add site-license delegates with admin user search	View Logs
membership package managers/ lets admins add site-license delegates with admin user search	View Logs
membership package managers/ lets admins add site-license delegates with admin user search	View Logs
membership package managers/ lets admins add site-license delegates with admin user search	View Logs
membership package managers/ lets admins add site-license delegates with admin user search	View Logs
membership package managers/ lets admins add site-license delegates with admin user search	View Logs
membership package managers/ lets admins add site-license delegates with admin user search	View Logs
membership package managers/ lets admins add site-license delegates with admin user search	View Logs
membership package managers/ lets admins add site-license delegates with admin user search	View Logs

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