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.

start.py

@@ -104,9 +104,10 @@ def main():
         subprocess.run(["npm", "install"], cwd=str(FRONTEND_DIR), check=True)
 
     subprocess.Popen(
-        ["npm", "run", "dev"],
+        "npm run dev",
         cwd=str(FRONTEND_DIR),
         creationflags=subprocess.CREATE_NO_WINDOW,
+        shell=True,
     )
     if not wait_port(5173):
         print("[FAIL] Frontend did not start in time.")