chore: release version 2.1.0-alpha02

[akshay2211]

Second 2.1 pre-release. Two additive seams for hosts that want to layer on
top of DrawBoxController (an intents SharedFlow and an overlay slot on
DrawBox), plus a fix for finger-drawn strokes on WASM/browser and for
translucent pen colours rendering as beaded dots.

No breaking changes on top of 2.1.0-alpha01. New surfaces stay under the
alpha stability bucket described in that release.

Added

• DrawBoxController.intents: SharedFlow<Intent> — observable stream of
  processed intents. Emission happens after the reducer runs and state has
  been updated, so subscribers can read state.value at emission time and
  see the resulting state. Unblocks layered controllers (brush), collaboration
  transports, and analytics recorders without subclassing or state-diffing.
  Buffered so slow subscribers don't back-pressure the drawing loop. (Open extension seams on DrawBoxController + DrawBox for brush and future collab v2 #99)
• overlay slot on DrawBox — new overlay: @Composable BoxScope.() -> Unit
  parameter, composed inside the canvas Box above strokes and selection
  chrome. Renders in screen space; hosts anchor to world coordinates via
  state.viewport. Default is a no-op, so existing call sites are
  unchanged. (Open extension seams on DrawBoxController + DrawBox for brush and future collab v2 #99)

Fixed

• Finger strokes halved on WASM/browser (Inconsistent line size #98). Pointer pressure is now
  trusted only from Stylus and Eraser input types. WASM/browser touch
  reports the W3C default 0.5 when there is no pressure sensor, which
  was silently halving finger strokes. Mouse and capacitive touch now fall
  back to unit pressure.
• Translucent pen strokes beading into dots (wrong lines when color has alpha channel  #97). The same touch
  pressure jitter was pushing uniform strokes into the variable-width
  branch, where per-segment round caps stacked alpha and beaded
  translucent colours. Also composites the variable-width path through
  canvas.saveLayer when alpha < 1, so a legitimate pen stroke with a
  translucent colour renders as a single translucent line instead of a
  chain of overlapping dots. (fix: ignore touch/mouse pressure and composite alpha strokes via layer #101)