feat(cli,mcp): idx stocks broker and get_broker_summary

`idx stocks broker SYMBOL` shows the top 10 buyers and sellers of a stock
by lots for the latest session with trades, one `--date`, or a
`--from`/`--to` range, filtered by `--investor` and `--board`. The MCP
server exposes the same as get_broker_summary.

- Range and filter parsing lives in idx-core (BrokerRange::from_args,
  FromStr for the filters), so both frontends accept the same input.
- Without dates, the latest session is found by walking back over
  weekdays; offline, uncached days are skipped.
- Table output lists foreign and state-owned brokers, and says when the
  source rounded figures.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V3syn2fN2LNo5pvTSqc6eh

flake.nix

@@ -288,7 +288,7 @@
                 echo '{"jsonrpc":"2.0","id":2,"method":"tools/list"}'
                 sleep 2
               } | timeout 30 ${mcpPackage}/bin/idx-mcp > out.jsonl
-              jq -se 'map(select(.id == 2))[0].result.tools | length == 11' out.jsonl
+              jq -se 'map(select(.id == 2))[0].result.tools | length == 12' out.jsonl
               touch "$out"
             '';
           # The website's generated pages match the CLI help and the MCP