Add support for shape filling to whiskers

[abey79]

This PR builds on the previous ones to introduce hatch fill support. This required refactoring the layer state handling, so we can have separate stroke and fill routing, replacing the single set_layer() method.

It also fixes per-path color/stroke_width defaults: paths now inherit from their layer instead of defaulting to BLACK/1.0, which caused layer colors to be overridden (e.g. crescent strokes appearing black instead of the layer's CMY color in quadri_circle).

Removed API

Method	Replacement
set_layer(layer_id)	stroke_layer(Some(layer_id))
color(c)	override_color(c) for per-path overrides, or layer(id).color(c) for layer defaults
stroke_width(w)	override_stroke_width(w) for per-path overrides, or layer(id).pen_width(w) for layer defaults

New API

Method	Description
layer(id) -> LayerHandle	Configure a layer's properties (pen width, color, name, hatch angle)
stroke_layer(Option<LayerID>)	Set which layer receives stroke output (None disables)
fill_layer(Option<LayerID>)	Set which layer receives fill/hatch output (None disables)
stroke_only(layer)	Shorthand: stroke on, fill off
fill_only(layer)	Shorthand: fill on, stroke off
stroke_and_fill(layer)	Shorthand: both on same layer
override_color(c)	Per-path color override (overrides layer default)
override_stroke_width(w)	Per-path stroke width override (overrides layer default)
clear_overrides()	Reset per-path overrides so paths inherit from layer
push_style() / pop_style()	Save/restore style state (layer routing + per-path overrides)
with_style(closure)	Scoped style changes with automatic restore

New traits: IntoBezPaths / IntoBezPathsTolerance

The Draw::add_path signature now accepts IntoBezPathsTolerance (plural) instead of IntoBezPathTolerance. This preserves structural boundaries when converting multi-geometry types (e.g. geo::MultiPolygon yields one BezPath per polygon), which is critical for correct per-shape hatching. Single-geometry types get a blanket impl wrapping the result in a one-element Vec, so existing code is unaffected.

New example

• fill_demo — demonstrates stroke/fill layer routing with a grid of shapes (circle, square, L-shape, polygon with holes) across three fill modes (none, horizontal, angled).

Default behavior change

Paths now default to None/None metadata (inheriting color and stroke width from their layer via resolve()). Previously, Sketch::new() set explicit BLACK/1.0 on every path, which silently overrode layer settings.

Migration guide

Setting color/pen width for a layer (most common case):

// Before
sketch.color(Color::DARK_RED).stroke_width(3.0);

// After — configure the layer directly
sketch.layer(0).color(Color::DARK_RED).pen_width(3.0);

Switching colors mid-drawing (per-path overrides):

// Before
sketch.color(Color::RED).circle(0., 0., 10.);
sketch.color(Color::BLUE).circle(0., 0., 20.);

// After — explicit override semantics
sketch.override_color(Color::RED).circle(0., 0., 10.);
sketch.override_color(Color::BLUE).circle(0., 0., 20.);
sketch.clear_overrides(); // back to layer defaults

Fill (hatching):

sketch.layer(1)
  .pen_width(0.5 * Unit::Mm)
  .hatch_angle(45.0_f64.to_radians());

sketch.stroke_layer(Some(0));
sketch.fill_layer(Some(1));
sketch.circle(100.0, 100.0, 50.0); // outline on layer 0, hatching on layer 1

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This is part 6 of 6 in a stack made with GitButler:

•  6  Add support for shape filling to whiskers #188 👈
•  5  Introduce IntoBezPaths trait #191
•  4  Add support for layer/path metadata inheritance #187
•  3  Add hatching algorithm to Path and FlattenedPath #186
•  2  Add .buffer() to Path and FlattenedPath #184
•  1  Add low-level implementation for path joining and merging optimization #183