improve lexical warmup and standardize stopword pipeline

modules/story-summary/vector/utils/stopwords-data/SOURCES.md

@@ -0,0 +1,15 @@
+# stopwords sources for story-summary
+
+- Dataset: `stopwords-iso` (npm package, version 1.1.0)
+- Repository: https://github.com/stopwords-iso/stopwords-iso
+- License: MIT
+- Snapshot date: 2026-02-16
+- Languages used: `zh`, `ja`, `en`
+- Local snapshot files:
+  - `stopwords-iso.zh.txt`
+  - `stopwords-iso.ja.txt`
+  - `stopwords-iso.en.txt`
+
+Generation note:
+- `modules/story-summary/vector/utils/stopwords-base.js` is generated from these snapshot files.
+- Keep `stopwords-patch.js` for tiny domain overrides only.