Feat: implement CTB/STB plot style table with PDF export integration

- src/io/plot_style.rs: PlotStyleTable and PlotStyleEntry data structures;
  CTB reader (deflate/zlib decompressed text format) with ACI→RGB and
  ACI→lineweight lookup; CTB writer (zlib compressed); LW_TABLE constant
- src/io/pdf_export.rs: accept optional PlotStyleTable; apply ACI color
  and lineweight overrides during PDF rendering
- src/scene/wire_model.rs: add `aci: u8` field to carry ACI index
- src/scene/render.rs: render_style() now returns ACI alongside RGBA
- src/scene/mod.rs + tessellate.rs: propagate aci through wire building
- app: PlotStyleLoad/Loaded/Clear messages and handlers;
  PLOTSTYLE LOAD | CLEAR | STATUS command; active_plot_style state on H7CAD
- flate2 added as dependency

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/io/mod.rs

@@ -4,6 +4,7 @@
 // Default save format: DWG (AC1032 / R2018+).
 
 pub mod pdf_export;
+pub mod plot_style;
 
 use acadrust::io::dwg::DwgReader;
 use acadrust::{CadDocument, DwgWriter, DxfReader, DxfWriter};
@@ -97,6 +98,21 @@ pub async fn pick_save_path() -> Option<PathBuf> {
     dlg.save_file().await.map(|h| h.path().to_path_buf())
 }
 
+// ── Plot Style Table ──────────────────────────────────────────────────────
+
+/// Show a file-open dialog and load the selected CTB or STB file.
+pub async fn pick_plot_style() -> Option<plot_style::PlotStyleTable> {
+    let handle = rfd::AsyncFileDialog::new()
+        .set_title("Load Plot Style Table")
+        .add_filter("Plot Style Tables", &["ctb", "stb", "CTB", "STB"])
+        .add_filter("CTB Files", &["ctb", "CTB"])
+        .add_filter("STB Files", &["stb", "STB"])
+        .add_filter("All Files", &["*"])
+        .pick_file()
+        .await?;
+    plot_style::PlotStyleTable::load(handle.path()).ok()
+}
+
 // ── Save ──────────────────────────────────────────────────────────────────
 
 /// Save the document to the given path.