fx_viewer: switch layout to fast-sugiyama with natural edge flow

Replace the vendored grandalf fork with the Rust-backed fast-sugiyama
package and rewrite the post-layout compaction in exporter.py to
produce a natural top-down edge flow.

Layout changes in _compute_layout_with_ext_lines / _compact_components:
- Drive layout via fast-sugiyama's from_edges + rect_pack_layouts.
- Run per-layer spine cohesion (chain detection across real and dummy
  nodes, iterative mean-pull, pure-A overlap repair) so linear op
  chains render as straight spines.
- Flip Phase 6's y so graph inputs / placeholders land at the top of
  the canvas and outputs / sinks at the bottom.
- Anchor edge endpoints to the bottom-midpoint of the source and
  top-midpoint of the target, giving each node a predictable dock.

Also refresh the README's dependency / layout section to point at
fast-sugiyama[all] and drop the grandalf install instructions.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: quic-boyuc

devtools/fx_viewer/README.md

@@ -6,7 +6,7 @@
 
 Python side:
 1. Extract FX graph (`torch.export` / `torch.fx`).
-2. Compute layout (Grandalf/Sugiyama).
+2. Compute layout (fast-sugiyama — Rust-backed Sugiyama).
 3. Build payload (`base` + `extensions`).
 4. Export JSON / JS snippet / standalone HTML.
 
@@ -15,6 +15,25 @@ JS side:
 2. Layer toggles and color-by controls.
 3. State-driven API for embedding, compare mode, fullscreen, and runtime layer mutation.
 
+## Dependencies
+
+Layout computation is delegated to the external
+[`fast-sugiyama`](https://github.com/austinorr/fast-sugiyama) package
+(Rust-backed, drop-in replacement for the previously vendored `grandalf`
+fork). Install with the `[all]` extra so that `rectangle-packer` is pulled
+in — it is required to pack disconnected graph components into a single
+non-overlapping layout:
+
+```bash
+pip install 'fast-sugiyama[all]'
+```
+
+- Python ≥ 3.11 is required by `fast-sugiyama`. Users still on Python 3.10
+  cannot use `FXGraphExporter`'s HTML/JSON output until they upgrade.
+- The package is **not** declared in executorch's `pyproject.toml`: it is
+  imported lazily on the first layout call. If it is missing, the exporter
+  raises an `ImportError` with install instructions.
+
 ## Quick Start
 
 From repo root:
@@ -37,18 +56,21 @@ from executorch.backends.qualcomm.utils.fx_viewer import (
     CategoricalColorRule,
 )
 
-exporter = FXGraphExporter(graph_module)
+# Step 1 instantiate exporter
+f = FXGraphExporter(graph_module)
 
+# Optional (define graph extension Layer)
 ext = GraphExtension(id="backend", name="Backend Assignment")
 ext.add_node_data("node_0", {"backend": "cpu"})
 ext.set_color_rule(CategoricalColorRule(attribute="backend"))
+f.add_extension(ext)
 
-exporter.add_extension(ext)
-exporter.export_html("graph.html")
+# Step 2 save standalone html
+f.export_html("graph.html")
 ```
 
 Main exporter methods:
-1. `generate_json_payload()`
+1. [ ] `generate_json_payload()`
 2. `export_json(path)`
 3. `export_js(container_id)`
 4. `export_html(path)`