fix(blocks): layer-0 children inherit the block instance layer (#221)

Entities inside a block definition that sit on layer "0" must take on the
properties of the layer the block reference is inserted on; every other
layer is "sticky" and keeps its own identity. OpenCADStudio resolved a
block child's ByLayer color/linetype/lineweight against the child's own
layer in all cases, so layer-0 content stayed layer-0 (white) instead of
inheriting the instance layer.

Resolution now follows the standard rule for block children — explicit
wins, then ByBlock → the insert's own style, then layer-0 + ByLayer →
the insert's *layer* style, else the child's own layer. The insert's
layer style threads through the whole block-expansion chain (and chains
correctly through nested inserts), with the child's own transparency
preserved.

Covered across every block-render path:
- wires (line/arc/circle/polyline/text) via the block cache + the
  explode fallback + insert attributes
- hatch / solid fills (also fixes the PDF/print export path)
- 3D solid block children (per-instance mesh recolor)
- Dimension / Table baked-block sub-entities

New shared helpers in scene::view::render: InheritStyle, layer_render_style,
lineweight_to_px; render_style_for_block_sub gained the layer-0 branch.
Adds unit tests covering the layer-0, sticky-layer, ByBlock, explicit and
transparency cases.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/entities/insert.rs

@@ -133,6 +133,7 @@ pub(crate) fn append_insert_attribute_wires(
     ins_pat_len: f32,
     ins_pat: [f32; 8],
     ins_lw_px: f32,
+    ins_layer: render::InheritStyle,
     bg_color: [f32; 4],
     is_xref: bool,
     pslt_factor: f32,
@@ -162,6 +163,7 @@ pub(crate) fn append_insert_attribute_wires(
             ins_pat_len,
             ins_pat,
             ins_lw_px,
+            ins_layer,
         );
         let sub_color = render::adapt_to_bg(sub_color, bg_color);
         let sub_color = if is_xref && !sel {