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.

kb_data/users.json

@@ -0,0 +1,12 @@
+[
+  {
+    "user_id": "d198ae3b32cd49f09736c4290dd1223a",
+    "name": "默认用户",
+    "created_at": "2026-05-23T12:19:43.785929+00:00"
+  },
+  {
+    "user_id": "2db10c2ebbf6434aab28035026e196c3",
+    "name": "SmokeTest",
+    "created_at": "2026-05-23T12:21:32.399217+00:00"
+  }
+]