feat(ls): add --match and --expand for dataset discovery

[djgormley]

Finding a dataset currently means walking the hierarchy one name at a time. This adds two options to ls:

• --match filters the larger datasets of a scope (or of every scope, when none is given) by comma-separated, case-insensitive terms that must all appear in the combined "scope dataset" text.
• --expand opens each matched larger dataset one level and lists the children with their parent, so a hit that is a container (e.g. complex_gains holding timestamped days) resolves to usable rows.

Both compose the existing functions.list REST calls; no new endpoints. A scope or dataset the server does not answer for is reported in a failed list (stderr warning, or a failed key with --json) rather than shown as empty; an empty map with unanswered queries exits 1. --expand without a scope or --match is refused, since it would open every dataset in the archive.

Docs updated; live tests for the new paths plus offline tests pinning the unanswered-vs-empty accounting.