fix(actions): keep small foreign dividends, allow only read paths, show open offers

From Pullfrog's review of #78:

- A dividend in another currency keeps 6 decimals (USD 0.0012 was rounded
  to 0.0 in JSON and "USD 0" in the table); rupiah stays to the sen.
- `--kind stock_dividend` and `--kind reverse_split` work on the CLI too,
  as they already did over MCP.
- The session client GETs only the prefixes in READ_PATHS, as
  docs/STOCKBIT.md's rule 1 already said: any other path (orders, PIN,
  withdrawals) fails before a request or token refresh leaves the process.
- `stocks calendar` and get_corporate_calendar also list rights trading,
  tender offers and IPO offers still open in the range, so an offer that
  began earlier shows while it can be acted on. A warrant's years-long
  windows do not count. The 2026-11-03..12 golden gains DOOH, NAYZ and AUTO.
- `stocks calendar --help` names the ex-date fallback, and docs/STOCKBIT.md
  shows the record and surface as shipped.

Claude-Session: https://claude.ai/code/session_01RqPPVeVDrnLoP7NUZmLRxY

crates/idx-mcp/src/server.rs

@@ -384,8 +384,10 @@ fields are omitted. One Stockbit request per kind (about 1 s each on a cold cach
 
     #[tool(
         description = "Corporate actions across IDX from the user's own Stockbit session, \
-oldest first, dated from `from` to `to` (default today and the next 30 days): upcoming cum \
-dates, meetings, exposes, rights, warrants, IPOs and tender offers, and the past year. Each \
+oldest first, dated from `from` to `to` (default today and the next 30 days), plus rights \
+trading, tender offers and IPO offers still open in that range (their `period` overlaps it, \
+though `date` may be earlier): upcoming cum dates, meetings, exposes, rights, warrants, IPOs \
+and tender offers, and the past year. Each \
 item has the same fields as get_corporate_actions, with `symbol`. One Stockbit request per \
 kind (about 1 s each on a cold cache, then cached 6 hours); pass `kinds` to ask for less. \
 AUTHERROR means there is no session: call stockbit_status, then stockbit_login."