feat(ui): saved trails — a walk you named, kept, and still true after a re-index (CG-60)

Save trail on the trail bar writes the walk to .codegraph/ui/trails/ as one
JSON file, listed on the empty screen and on Entry points above the derived
suggestions, reopened at the symbol you left with the whole path restored.

A hop is stored by qualified name, kind and file — never by node id, which
contains a start line and so changes the first time anybody edits above the
symbol. Every hop is re-resolved against the current index on the way out and
each row says what became of it: still here, moved to another file, now
ambiguous, or gone. A hole is never stitched over: the row opens the longest
run of CONSECUTIVE resolved hops and says which ones those are, because the
trail is a path and a skipped hop would draw a call that does not exist.

This is the first write the viewer makes, and the boundary moved with it:
POST/DELETE answer under /api/ only, must carry X-CodeGraph-UI and
application/json (neither of which a cross-origin form can produce without a
preflight this server answers none of), and --read-only refuses both while
still listing what is there. The blanket "read-only" claim is retired from the
banner, the README, the CLI help and the docs site in favour of the narrower
true one.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/index.ts

@@ -1373,6 +1373,21 @@ export class CodeGraph {
     return this.queries.getNodesByIds(ids);
   }
 
+  /**
+   * Every symbol carrying an exact qualified name.
+   *
+   * The identity that survives a re-index. A node's id contains its start line,
+   * so any edit ABOVE a symbol gives it a different id — anything that has to
+   * name the same symbol across two indexes (a saved trail, a bookmark, a
+   * review comment) has to key on this instead, and then disambiguate the
+   * result by kind and file. Index-backed; unlike
+   * {@link GraphQueryManager.findByQualifiedName} it takes no pattern and scans
+   * nothing.
+   */
+  getNodesByQualifiedName(qualifiedName: string): Node[] {
+    return this.queries.getNodesByQualifiedNameExact(qualifiedName);
+  }
+
   /**
    * Outgoing edges for many source nodes at once — the batch form of
    * {@link getOutgoingEdges}. See {@link QueryBuilder.getOutgoingEdgesFrom}.