Support uninstalled / missing nodes

Previously the index was built only from the live registry, so a custom node
that wasn't installed (a red "missing" node in a downloaded workflow) was
invisible — the main point of the tool. Now:

- Backend: utfcn_core split into build_context / build_index / match. build_index
  also emits curated candidates for uninstalled source types (curated-only), and
  a new POST /utfcn/match matches missing nodes by their serialized signature
  against installed core/other-pack nodes.
- Frontend: nodeType() reads a missing placeholder's last_serialization.type;
  matchMissing() feeds serialized slots to /utfcn/match and merges the results;
  the right-click item moved to a canvas-level getNodeMenuOptions patch so it
  reaches missing placeholders too. Bulk dialog labels them "not installed".

Replace a missing node with core without installing its pack. Links are rewired
losslessly; widget values can't be carried for a node whose def is absent.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -23,6 +23,17 @@ It does three things:
 Nothing is ever swapped without your say-so, and the engine only rewires slots
 it can move *losslessly* — anything it can't map is reported, not guessed.
 
+### Works on uninstalled ("missing") nodes
+
+The headline case: open a downloaded workflow full of red **missing** nodes and
+replace them with core equivalents **without installing the packs at all**.
+ComfyUI keeps each missing node as a placeholder that remembers its original
+type and wiring, so UTFCN can still match and swap it — via a curated rule (by
+name) or by matching the node's *serialized* signature against your core nodes.
+Both "Replace…" and the right-click item work on them; the bulk dialog labels
+them `⚠ not installed`. (Widget values aren't carried for a node whose
+definition you don't have — links are.)
+
 ## How it decides what's equivalent
 
 The backend reads the live node registry (real `INPUT_TYPES` / `RETURN_TYPES`
@@ -37,6 +48,10 @@ and each node's source module) and ranks candidates in three tiers:
 "Available" means core is preferred, and if there's no core match it will offer
 an equivalent from a **different installed pack** as a fallback.
 
+For an **uninstalled** node only *curated* (by name) and *partial* (by its
+serialized link signature) can apply — the exact tier needs the widget-level
+signature, which a node you haven't installed can't provide.
+
 ## Shipped equivalences
 
 `mappings.json` ships a small, hand-verified set (each checked lossless against