fix(library): a roll picked at a drive root counts its frames under one spelling

The picker hands a whole drive over named `\`, which `normPath` folds to the
empty root, and a row of that roll was keyed `Roll` where the reading files its
counts, its walked folders and its frames under `/Roll` — one folder under two
names. Every number beside a subfolder of a picked drive read nought, and the
spinner of a reading stood on the roll's own row and nowhere it was reading.

The row key is the roll's own spelling again, and so is the path `relOf` hands
back for a row under it: a reading kept to one folder of a drive root asks for
`/Roll`, and `sub` counted from the old `Roll/` merged nothing. Both are the
spelling the check at the foot of `scripts/tree-scan-check.mjs` asks for, and it
is green again.

A bounded reading also measured itself against `\/Roll` spelled from the raw
name, which no id is filed under: `held` stood at nought and the line read the
catalogue's whole roll as the ground it was climbing from. It is built from
`normPath` of both, the way an id is.

docker/frontend/src/Library.tsx

@@ -1586,7 +1586,6 @@ function Catalogue({ admin, user }: { admin: boolean; user: User | null }) {
       const normRoot = normPath(rootName);
       const normKey = normPath(key);
       if (normKey === normRoot) return '';
-      if (normRoot === '') return `${normKey}/`;
       if (normKey.startsWith(`${normRoot}/`)) return `${normKey.slice(normRoot.length + 1)}/`;
       return null;
     },
@@ -1766,7 +1765,7 @@ function Catalogue({ admin, user }: { admin: boolean; user: User | null }) {
       // indented rows have to come in.
       for (const dir of [...(foldersUnder.get(lowerF) ?? [])].sort((a, b) => a.localeCompare(b))) {
         const parts = dir.split('/');
-        const key = normF ? `${normF}/${dir}` : dir;
+        const key = `${normF}/${dir}`;
         out.push({
           key,
           label: parts[parts.length - 1],

docker/frontend/src/engine/library.ts

@@ -1390,7 +1390,11 @@ export async function scanFolder(
   // been read once before is a reading of a hundred thousand frames even while
   // the walk is still up in its first folders, and one kept to a subfolder is
   // measured against the frames under that one alone.
-  const heldPrefix = `${folder.name}/${from}`.toLowerCase();
+  // An id is `normPath(folder.name)/…`, and a drive root is named `\`, which
+  // `normPath` spells `''` — so the frames of such a roll's subfolder are filed
+  // under `/Roll/…`, and a prefix built as `${folder.name}/${from}` spells
+  // `\/Roll` and counts nought of them held.
+  const heldPrefix = `${normPath(folder.name)}/${normPath(from)}`.toLowerCase();
   const held = bounded
     ? rawKnown.filter((p) => p.id.toLowerCase().startsWith(heldPrefix)).length
     : rawKnown.length;