Docs: rewrite search-by-meaning around a vec0 index instead of per-row embed() #1003

Problem

docs/howto/search-by-meaning.md teaches semantic search as a path-table
subquery that embeds every matched file at query time:

SELECT path,
       vec_distance_cosine(emb, embed('how do I cook pasta?')) AS distance
FROM (SELECT path, embed(content) AS emb FROM './notes/*.md')
WHERE emb IS NOT NULL
ORDER BY distance
LIMIT 3

That is the only shape the guide teaches, and it costs one worker round trip
per matched file, per query, on top of a path-table walk that re-reads every
file's content. The vector cache
removes the model inference on a repeat run — it does not remove the round
trip and it does not remove the walk.

Since #956 the alternative is expressible in config: a vec0 virtual table
declared in the same ddl batch as the row table, filled by a trigger calling
embed(). A query then embeds once (the query text) and vec0 does an
in-process KNN scan — the query-side effect #956 called out.

#994 added a ::: tip pointing at docs/reference/config.md#batch-ddl, but
left the guide's central example, and its whole narrative, on the per-row
shape.

Change

Rewrite the guide so the declared-table + vec0 shape is the primary path:

1. Lead with the [[table]] + vec0 + embed() trigger config (the vector
   template in Docs: path-table vs declared-table tradeoff; FTS5 and vector index templates #960).
2. Query with WHERE embedding MATCH embed(:q, '') AND k = :n, joined back to
   the base table on rowid, ordered by distance.
3. Keep the path-table ORDER BY vec_distance_cosine(...) LIMIT k shape as the
   zero-setup option and state the trade explicitly — it is the same
   declared-table-vs-path-table trade Docs: path-table vs declared-table tradeoff; FTS5 and vector index templates #960 documents: no config, always
   current, pays per query; versus config plus --persist, watcher-maintained,
   one embed per query.
4. Retire or rewrite the ::: tip Top-k is LIMIT k block. It exists to explain
   why MATCH … AND k = does not apply, which stops being true once the
   primary example uses vec0.
5. Note what the trigger shape inherits from Declared tables: wide-open multi-statement ddl, catalog validation #956: the model id lives in the
   ddl string, so switching models changes the config hash and forces a full
   re-embed. No stale vectors, no ownership tracking.

Depends on #960 for the vector template (or supersedes that half of it).
Related: #956, #994.

Size: S. Docs only.