feat: activity sidecar edits via bincio edit --serve

- bincio/render/merge.py: parse sidecar .md files (YAML frontmatter +
     markdown body), produce data/_merged/ with symlinks for unmodified
     activities and real merged files for overridden ones; filters private
     activities from index.json; sorts highlighted activities first.
     Keeps extracted data pristine — re-running extract never clobbers edits.

   - bincio/edit/: FastAPI edit server (port 4041) with embedded HTML/JS
     edit UI; GET/POST /api/activity/{id} reads/writes sidecars; multipart
     image upload to edits/images/{id}/; DELETE for image cleanup.

   - bincio render now calls merge_all() before build/serve and symlinks
     public/data → data/_merged/ instead of data/ directly.

   - ActivityDetail.svelte: edit button (links to edit server) when
     PUBLIC_EDIT_URL env var is set; respects custom.hide_stats to suppress
     stat panels; description supports whitespace-preserving rendering.

   - 15 unit tests covering parse_sidecar, apply_sidecar, and merge_all.

bincio/render/cli.py

@@ -61,11 +61,23 @@ def _ensure_npm(site: Path) -> None:
         subprocess.run(["npm", "install"], cwd=site, check=True)
 
 
+def _merge_edits(data: Path) -> None:
+    """Run the sidecar merge step, producing data/_merged/."""
+    from bincio.render.merge import merge_all
+    n = merge_all(data)
+    if n:
+        console.print(f"Merged [cyan]{n}[/cyan] sidecar edit(s) into _merged/")
+    else:
+        console.print("No sidecars found — _merged/ mirrors extracted data.")
+
+
 def _link_data(site: Path, data: Path) -> None:
-    """Symlink the BAS data store into site/public/data."""
+    """Symlink site/public/data → data/_merged/ (the post-merge output)."""
+    merged = data / "_merged"
+    target = merged if merged.exists() else data
     public_data = site / "public" / "data"
     if public_data.is_symlink():
-        if public_data.resolve() == data:
+        if public_data.resolve() == target.resolve():
             return  # already correct
         public_data.unlink()
     elif public_data.exists():
@@ -74,8 +86,8 @@ def _link_data(site: Path, data: Path) -> None:
             "remove it manually if you want bincio to manage it."
         )
         return
-    public_data.symlink_to(data)
-    console.print(f"Linked data: [cyan]{data}[/cyan] → [cyan]{public_data}[/cyan]")
+    public_data.symlink_to(target)
+    console.print(f"Linked data: [cyan]{target}[/cyan] → [cyan]{public_data}[/cyan]")
 
 
 @click.command()
@@ -108,6 +120,7 @@ def render(
     console.print(f"Data:  [cyan]{data}[/cyan]")
 
     _ensure_npm(site)
+    _merge_edits(data)
     _link_data(site, data)
 
     env = {**os.environ, "BINCIO_DATA_DIR": str(data)}