feat(ui): add ui.toast, a message that slides up from the bottom edge

The card sits in a scroll container of its own height over an empty strip of
the same, so the slide is a setScroll() delta on the laid-out tree rather than
a rebuild a frame, and the container scissors the part not yet arrived.

lua/lib/ui/toast.lua

@@ -0,0 +1,94 @@
+-- A message that slides up from the bottom edge, holds, then slides back down.
+--
+-- The card sits inside a scroll container of its own height whose content is twice that:
+-- an empty strip above the card. Sliding is therefore ui.setScroll() on the tree already
+-- laid out -- a delta on stored coordinates -- rather than a rebuild a frame, and the
+-- container scissors the half of the card that has not arrived yet.
+--
+-- Whatever mounts the tree includes toast.node() and calls toast.draw(deltaMs); show()
+-- rebuilds so the node exists, and hide() rebuilds to drop it.
+
+local ui = require "ui"
+
+local M = {}
+
+local HEIGHT = 44
+local MARGIN = 12
+local SLIDE_MS = 180
+
+local text, box
+-- nil, or "in" / "hold" / "out" with the milliseconds spent in it.
+local phase, elapsed, holdMs
+
+---@param message string
+---@param durationMs? integer How long the card holds before it slides away.
+function M.show(message, durationMs)
+  text, holdMs = message, durationMs or 1500
+  phase, elapsed = "in", 0
+  ui.rebuild()
+end
+
+function M.hide()
+  if phase then
+    phase, text, box = nil, nil, nil
+    ui.rebuild()
+  end
+end
+
+---@return NodeId|nil
+function M.node()
+  if not phase then
+    return nil
+  end
+  box = ui.box {
+    at = { x = MARGIN, y = screen.getHeight() - HEIGHT - MARGIN },
+    w = screen.getWidth() - 2 * MARGIN,
+    h = HEIGHT,
+    scrollY = true,
+    ui.box {
+      w = "fill",
+      ui.spacer { w = "fill", h = HEIGHT },
+      ui.box {
+        w = "fill",
+        h = HEIGHT,
+        align = "center",
+        justify = "center",
+        background = ui.theme.face,
+        border = ui.theme.muted,
+        ui.text(text),
+      },
+    },
+  }
+  return box
+end
+
+local function progress()
+  if phase == "in" then
+    return elapsed / SLIDE_MS
+  elseif phase == "out" then
+    return 1 - elapsed / SLIDE_MS
+  end
+  return 1
+end
+
+---Advances the slide. Safe to call every frame whether or not a toast is up.
+---@param deltaMs integer
+function M.draw(deltaMs)
+  if not phase then
+    return
+  end
+  elapsed = elapsed + deltaMs
+  if phase == "in" and elapsed >= SLIDE_MS then
+    phase, elapsed = "hold", 0
+  elseif phase == "hold" and elapsed >= holdMs then
+    phase, elapsed = "out", 0
+  elseif phase == "out" and elapsed >= SLIDE_MS then
+    M.hide()
+    return
+  end
+  if box then
+    ui.setScroll(box, 0, math.floor(HEIGHT * progress()))
+  end
+end
+
+return M