fix(core): symmetric bandar thresholds; bandar needs all investors

- Basis points truncate toward zero instead of flooring, so a reading of
  -4.9952% stays neutral like its mirror image and -14.9952% stays
  distribution; tests cover both signs at both thresholds through
  bandar_tier. Goldens move by the rounding (BBCA top 5: -2426 -> -2425).
- With one investor side selected the summary has no total to weigh
  brokers against, so bandar and analyze_bandar reject it (INVALIDINPUT)
  instead of reading every tier as missing.

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

crates/idx-mcp/src/server.rs

@@ -205,7 +205,8 @@ traded (`imbalance_bps`), with a `signal` from big_accumulation (>= +1500) throu
 (within +-500) to big_distribution (<= -1500), headlined by `top` (default 5). Reports come \
 strongest accumulation first; tickers that fail are listed under `failures`. A heuristic over \
 end-of-day broker codes: brokers trade for many clients, and `both_sides` lists brokers whose \
-gross figures overstate their direction. `method` names the rules applied."
+gross figures overstate their direction. `method` names the rules applied. `investor` must be \
+`all`: the summary has no total for one investor side."
     )]
     async fn analyze_bandar(&self, params: Parameters<BandarParams>) -> ToolResult {
         let BandarParams {