fix: address final review (path traversal, sync races, MCP limits)

Findings from a Codex (gpt-6.1-sol, xhigh) review of v0.2.4..HEAD, each
verified in the code before fixing:

- high: ticker symbols reached cache file paths unchecked, so a symbol
  like `../../REPORT` read (and deleted, if unparseable) .json files
  outside the cache, reachable through MCP get_quotes. resolve_symbol now
  accepts only ticker characters, and cache keys are encoded as single
  safe path components.
- high: sync decided before downloading and replaced the database
  without rechecking; an import that landed meanwhile was erased. It now
  rechecks the local state before installing, and idx-mcp runs
  ownership_sync exclusively with ownership reads.
- replaces_sqlite_sha256 never downgrades history: the client requires
  the rebuild to keep at least the local release count, and the
  publisher lists a replaced build only then.
- cache writes are atomic (temp file + rename), so concurrent readers
  never see, and delete, a partial entry.
- idx-mcp caps symbols (20, deduplicated) and limits, and runs at most 4
  tool calls at once.
- each Yahoo client gets its own cookie jar, paired with its crumb.
- ZIP entries (XLSX, KSEI archives) are read with a 256 MiB bound;
  non-ASCII archive dates and huge percentages are parse errors instead
  of panics or overflow.
- agent-smoke --keep deletes every copied agent config, not only auth
  files.

CLI goldens unchanged; the MCP tool golden changes only to state the
new caps.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/idx-core/src/ownership/entities.rs

@@ -81,7 +81,10 @@ pub fn parse_id_percentage(s: &str) -> Result<i64, IdxError> {
         IdxError::ParseError(format!("failed to parse Indonesian percentage '{s}': {e}"))
     })?;
 
-    Ok((whole_i * 100) + frac_i)
+    whole_i
+        .checked_mul(100)
+        .and_then(|bps| bps.checked_add(frac_i))
+        .ok_or_else(|| IdxError::ParseError(format!("Indonesian percentage '{s}' is out of range")))
 }
 
 /// Parse KSEI date format to NaiveDate.
@@ -453,4 +456,10 @@ mod tests {
 
         assert!(normalize_ksei_row(&raw).is_err());
     }
+
+    #[test]
+    fn huge_percentages_are_parse_errors_not_overflow() {
+        assert!(super::parse_id_percentage("9223372036854775807,99").is_err());
+        assert_eq!(super::parse_id_percentage("54,94").unwrap(), 5494);
+    }
 }