feedback5 WU: module type icons get one lower-case file per type (dec-X and dec-x were two names for one file)

The official data spells one DEC X module 'dec-X' (uniequip_003_aglina) and the others 'dec-x' (WAH-Y is the other
mixed-case type id), and the asset plan stored each under its own case: module/dec-X.png and module/dec-x.png. On
Windows / macOS those are one file, so a release zip built there could hold only one of them and a case-sensitive
server (a Linux VPS) would miss the other — the release packager found it ("planned but not staged"). The local file
is now module/{typeIcon in lower case}.png; the upstream names keep their order (the id as spelled, then lower case).

data/assets.json regenerated offline (node tools/fetch-assets.mjs --offline --add-only). JSON diff: modules.dec-X
/assets/module/dec-X.png -> /assets/module/dec-x.png, modules.WAH-Y /assets/module/WAH-Y.png -> /assets/module/wah-y.png,
hash, stats.files 6979 -> 6978, stats.bytes 458334505 -> 458333444; nothing else.

fetch-assets compares its orphans (what --prune deletes) without case: on Windows / macOS module/WAH-Y.png on disk is
the listed module/wah-y.png and must not be deleted (orphanFiles, exported).

Tests: test/assets-diy.test.js (one lower-case file per type, no two manifest URLs differing only in case, orphans).
docs/ASSETS.md: the module icon file name.

tools/fetch-assets.mjs

@@ -158,6 +158,19 @@ export function shrinkGuard(prev, next, { allowShrink = false, prune = false } =
   return { dropped, write: dropped.length === 0 || !!allowShrink || !!prune };
 }
 
+/**
+ * Files under public/assets the manifest does not reference (`--prune` deletes them). public/assets/local/** belongs to
+ * tools/local-extract (data/local-assets.json) and is never an orphan: --prune used to delete all of it. Compared without
+ * case: on Windows / macOS a listed path and a file whose name differs only in case are one file (module/WAH-Y.png on
+ * disk serves the listed module/wah-y.png), which --prune must not delete.
+ * @param {string[]} onDisk forward-slash paths relative to public/assets
+ * @param {Iterable<string>} referenced the manifest's files, same form
+ */
+export function orphanFiles(onDisk, referenced) {
+  const listed = new Set([...referenced].map((r) => r.toLowerCase()));
+  return onDisk.filter((r) => !listed.has(r.toLowerCase()) && !r.startsWith('local/'));
+}
+
 const mb = (n) => `${(n / 1048576).toFixed(1)} MB`;
 const log = (m) => console.log(m);
 
@@ -363,9 +376,8 @@ async function main() {
   const guard = shrinkGuard(current, manifest, opts);
   if (guard.write) await writeJsonAtomic(MANIFEST, manifest);
 
-  // Orphans: files on disk that the manifest does not reference (e.g. after a mapping change). public/assets/local/**
-  // belongs to tools/local-extract (data/local-assets.json) and is never an orphan: --prune used to delete all of it.
-  const orphans = (await listFiles(ASSETS)).filter((r) => !resolved.files.has(r) && !r.startsWith('local/'));
+  // Orphans: files on disk that the manifest does not reference (e.g. after a mapping change).
+  const orphans = orphanFiles(await listFiles(ASSETS), resolved.files);
   if (opts.prune) for (const r of orphans) { try { await unlink(join(ASSETS, r)); } catch { /* ignore */ } }
 
   const charIds = Object.keys(assets07.operators || {});