CLI: show Observable signal stats in `record list` table output

[lxsaah]

Records registered with .observe() publish live domain-signal gauges
(last/min/max/mean plus a unit) and the AimX protocol already delivers them:
RecordMetadata.signal_stats is populated when the observability feature is
on and the CLI enables that feature, so aimdb record list --format json
includes the stats today. But the default human-readable table drops them: it
only prints Name, Type ID, Buffer Type, Producers, Consumers and Writable.
The most useful live information the protocol carries never reaches the
terminal.

Where to look

• tools/aimdb-cli/src/output/table.rs — format_records_table, the columns to extend (there's a unit test at the bottom of the file to grow with it)
• aimdb-core/src/remote/metadata.rs — RecordMetadata.signal_stats (Option<Vec<SignalStatsInfo>>)
• tools/aimdb-cli/src/commands/record.rs — list_records, where table vs JSON output is chosen
• cargo run -p remote-access-demo --bin server — a live test instance at /tmp/aimdb-demo.sock; its Temperature record is registered with .observe(), so aimdb record list --format json shows its gauge and the other records make good - cases

What to do

• Extend format_records_table with a compact signal column, e.g.
  Signal rendering last unit (min…max, ⌀mean) — records without gauges
  show a dimmed -. Keep the table readable at typical terminal widths;
  a record can carry more than one gauge, so pick a sensible compact form
  (first gauge + count, or one line per gauge).
• No feature-gating needed on your side: the CLI always builds aimdb-core
  with observability on, so signal_stats is a regular Option field —
  match on Some/None and render the dimmed - for None.

Done when

• aimdb record list against the remote-access-demo server shows live
  stats for server::Temperature and - for the rest
• JSON output is unchanged
• make check is green