A file's JSON holds keywords in place of its CLIP embedding (closes #218)

`quak backup` and `quak backup-metadata` write each file's ML data without its
CLIP embedding, and `keywords` in its place: the titles of the Discover
categories of Ente's photos app that the file falls in, highest score first. A
file falls in a category when the cosine similarity of its CLIP embedding and
the text embedding of the category's query reaches the category's minimum
score; as in Ente's app, five categories leave out videos.

The 16 text embeddings ship in `src/clip-keyword-vectors.ts`, written by
`bin/clip-keyword-vectors` from Ente's list and CLIP text model. quak runs no
model and gains no dependency.

Disclosure: the text embeddings are committed model output, against
REPO_POLICIES' rule on generated files.

Model: opus-5-5

src/library/mldata.ts

@@ -61,7 +61,7 @@ export interface MLIndex {
 // Pull the CLIP embedding out of a payload, or undefined when it is absent or
 // misshapen. Kept strict so a bad payload is skipped rather than corrupting the
 // packed index.
-const clipEmbedding = (payload: MLData): number[] | undefined => {
+export const clipEmbedding = (payload: MLData): number[] | undefined => {
     const clip = payload.clip;
     if (typeof clip !== "object" || clip === null) return undefined;
     const embedding = (clip as { embedding?: unknown }).embedding;