fix: band-level windowed refine_layout + programmatic map_fields to prevent 91.5% content loss

Root cause: LLM receiving full 34k-char JRXML would regenerate from scratch
instead of modifying coordinates in-place, shrinking output to ~3k chars.

Solution (programmatic node control, not prompt engineering):

- New agent/jrxml_windower.py: decompose JRXML into header (never sent to
  LLM) + individual bands. Split bands >4000 chars at element boundaries.
  Reassemble with element count validation (>10% change = rollback).

- Rewrite refine_layout: per-band windowed LLM processing (~2-4k chars
  each). LLM cannot "reimagine" the entire report.

- Rewrite map_fields: 100% programmatic regex $F{field_N} -> real name
  replacement. Zero LLM calls, zero content loss.

- _sanitize_field_name: non-ASCII chars escaped to _uXXXX_ format for
  valid JRXML identifiers.

- Tests: 48 new unit tests (windower 28 + map_fields 20). All passing.
  Full suite 385 tests, zero regressions.

backend/session.py

@@ -56,6 +56,7 @@ def create_session(name: str = "", agent_state: Optional[dict] = None,
         "session_name": name or f"新建报表 {now[:10]}",
         "created_at": now,
         "updated_at": now,
+        "kb_id": agent_state.get("kb_id", "") if agent_state else "",
         "agent_state": agent_state,
     }
     with open(_session_path(sid), "w", encoding="utf-8") as f: