feat(export): DOCX renderer via docx-rs (T8.4, FR-NOTE-4)

Bullet items get a literal "• " prefix rather than a real Word
numbered-list definition — this is read-only generated content, not
something the user continues typing into, so numbering-continuity
machinery isn't worth the extra API surface.

src-tauri/src/notes/docx.rs

@@ -0,0 +1,60 @@
+//! Word (.docx) export (Phase 8, FR-NOTE-4) via docx-rs — no external
+//! binary/cloud conversion, consistent with the fully-local invariant.
+
+use super::{Block, NotesError, Span};
+use docx_rs::{Docx, Paragraph, Run};
+use std::path::Path;
+
+pub(super) fn render(blocks: &[Block], dest: &Path) -> Result<(), NotesError> {
+    let mut docx = Docx::new();
+    for block in blocks {
+        docx = docx.add_paragraph(to_paragraph(block));
+    }
+
+    if let Some(parent) = dest.parent() {
+        std::fs::create_dir_all(parent).map_err(NotesError::Io)?;
+    }
+    let file = std::fs::File::create(dest).map_err(NotesError::Io)?;
+    docx.build()
+        .pack(file)
+        .map_err(|e| NotesError::Export(format!("docx save: {e}")))?;
+    Ok(())
+}
+
+fn to_paragraph(block: &Block) -> Paragraph {
+    match block {
+        Block::Heading(level, spans) => {
+            let size = match level {
+                1 => 32,
+                2 => 28,
+                _ => 24,
+            };
+            let mut p = Paragraph::new();
+            for s in spans {
+                p = p.add_run(Run::new().add_text(s.text.as_str()).bold().size(size));
+            }
+            p
+        }
+        Block::Paragraph(spans) => spans_to_paragraph(spans, ""),
+        // ponytail: a literal "• " prefix, not a real Word numbered-list
+        // definition (AbstractNum/Num) — this is read-only generated
+        // content, not something the user continues typing into, so
+        // auto-numbering continuity isn't worth the extra API surface.
+        Block::BulletItem(spans) => spans_to_paragraph(spans, "\u{2022} "),
+    }
+}
+
+fn spans_to_paragraph(spans: &[Span], prefix: &str) -> Paragraph {
+    let mut p = Paragraph::new();
+    if !prefix.is_empty() {
+        p = p.add_run(Run::new().add_text(prefix));
+    }
+    for s in spans {
+        let mut run = Run::new().add_text(s.text.as_str());
+        if s.bold {
+            run = run.bold();
+        }
+        p = p.add_run(run);
+    }
+    p
+}