observatory: default graph compare sync to sparse_match_key for per-layer accuracy

Add `default_sync` field to `GraphCompareSpec` so lenses can declare
their preferred compare sync strategy. The per-layer accuracy lens now
defaults to `mode: "layer"` on `sparse_match_key` instead of the
generic auto mode, giving immediate node correspondence when comparing
records side-by-side.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: quic-boyuc

devtools/fx_viewer/templates/README.md

@@ -94,6 +94,8 @@ layout.container
 | `'layer'` | Ext: \<extId\>.\<field\> | Matches by `extensions[layer].nodes[nodeId].info[field]` value equality; picks last in topo order on multiple matches |
 | `'none'` | Don't sync | No cross-viewer selection propagation |
 
+The initial sync mode can be set programmatically via `config.sync` at construction time. Observatory's `GraphCompareSpec.default_sync` maps directly to this field, allowing lenses to declare their preferred default sync strategy.
+
 #### `debug_handle` normalization (`mode: 'auto'`)
 
 `debug_handle` in `node.info` is `int` (scalar) or `int[]` (list, for fused nodes). The sync engine normalizes both to a `Set<int>` and uses **set intersection** to find matches:

devtools/observatory/REFERENCE.md

@@ -447,9 +447,10 @@ Fields:
 1. `mode: "auto" | "disabled" | "custom"`
 2. `max_parallel: int >= 1`
 3. `sync_toggle: bool`
-4. `viewer_options_compare: dict`
-5. `js_func: str | None`
-6. `args: dict`
+4. `default_sync: dict` — initial `FXGraphCompare` sync config passed as `config.sync`. Keys: `mode` (`"auto"`, `"id"`, `"layer"`, `"none"`), `layer` (extension id), `field` (info key). Empty dict (default) falls back to `{ mode: "auto" }`.
+5. `viewer_options_compare: dict`
+6. `js_func: str | None`
+7. `args: dict`
 
 #### 2. Validation API
 

devtools/observatory/interfaces.py

@@ -189,6 +189,7 @@ class GraphCompareSpec:
     mode: Literal["auto", "disabled", "custom"] = "auto"
     max_parallel: int = 2
     sync_toggle: bool = True
+    default_sync: Dict[str, str] = field(default_factory=dict)
     viewer_options_compare: Dict[str, Serializable] = field(default_factory=dict)
     js_func: Optional[str] = None
     args: Dict[str, Serializable] = field(default_factory=dict)

devtools/observatory/lenses/per_layer_accuracy.py

@@ -29,6 +29,7 @@
 from ..interfaces import (
     AnalysisResult,
     Frontend,
+    GraphCompareSpec,
     GraphView,
     HtmlBlock,
     HtmlRecordSpec,
@@ -811,6 +812,13 @@ def record(
                     graph_ref=graph_ref,
                     default_layers=[f"{lens_name}/cosine_sim"],
                     default_color_by=f"{lens_name}/cosine_sim",
+                    compare=GraphCompareSpec(
+                        default_sync={
+                            "mode": "layer",
+                            "layer": f"{lens_name}/cosine_sim",
+                            "field": "sparse_match_key",
+                        }
+                    ),
                     order=21,
                 ).as_block(),
                 HtmlBlock(

devtools/observatory/templates/js/03_blocks.js

@@ -436,7 +436,9 @@
     const compare = CompareCtor.create({
       viewers: viewerMap,
       layout: { container: content },
-      sync: { mode: 'auto' },
+      sync: compareSpec.default_sync && compareSpec.default_sync.mode
+        ? compareSpec.default_sync
+        : { mode: 'auto' },
     });
 
     state.graphCompareInstances.set(cacheKey, { compare, nameToIndex });