feat(lapaloma-financial): OHLC + candlesticks con preserva-volatilidad

- axis.rs: paint_axes extraído a función pública reusable entre
  crates de visualización. LapalomaChartElement::paint_axes ahora
  es un thin wrapper.
- OhlcBuffer: stride 6 f32 por bar (t, o, h, l, c, v). Bar struct
  con is_bull/is_bear. price_range y time_range. 5 tests.
- aggregate_time_bucketed (sección 3.2 del ARCHITECTURE.md):
  buckets por TIEMPO (no índice) — open=first, close=last,
  high=max, low=min, volume=sum. Preserva volatilidad (los wicks
  sobreviven al downsample, a diferencia de LTTB). Fallback a
  copy 1:1 si el span temporal es cero. 4 tests cubren bucket
  count, preservation of volatility, fallback, empty input.
- paint_candlesticks: render agnóstico contra el trait Canvas.
  Wick = stroke_line vertical (high → low). Body = fill_rect
  open ↔ close con color bull/bear/neutral. body_width derivado
  del spacing entre bars (con body_min_width floor).
- LapalomaCandlestickElement: Element GPUI que reusa paint_axes
  + paint_candlesticks. Sin pan-blit cache en v0.1 (≤500 bars
  on-screen no lo necesita).
- crates/apps/lapaloma-financial-demo: random walk determinístico
  (xorshift32 inline + seed fijo) de 120 bars, pan + zoom + reset
  igual que el cartesian demo. Paleta nórdica para bull (#a3be8c)
  y bear (#bf616a).

60 tests verdes (28 cartesian + 20 core + 9 financial + 3 render).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/modules/ui_engine/widgets/lapaloma-financial/Cargo.toml

@@ -11,4 +11,8 @@ description = "Lapaloma — gráficos financieros. OHLC / candlesticks con agreg
 lapaloma-core = { path = "../../libs/lapaloma-core" }
 lapaloma-render = { path = "../lapaloma-render" }
 lapaloma-cartesian = { path = "../lapaloma-cartesian" }
-gpui = { workspace = true }
+gpui = { workspace = true, optional = true }
+
+[features]
+default = ["gpui"]
+gpui = ["dep:gpui", "lapaloma-render/gpui", "lapaloma-cartesian/gpui"]