Locate Keihin tables + render real fuel maps in the viewer

Reversed the inner map format from l.java (Nc/Lb) and validated it against the
stock reference maps:

- reconstruct_rom.py: decoded map -> unpack directory -> 384KB flat ROM. In the
  flat ROM, production (20187) vs aftermarket (20188) differ only 1.4% (vs 96%
  packed), i.e. real fuel enrichment.
- table_map.py + TABLES.md: fe = c.a[Qd*48]; base = (fe[0]&0x2F0)<<12 = 0x50000;
  table = base + fe[k]. Located 9 tables (main/low-throttle fuel per cylinder,
  base/idle fuel, ignition by gear x4), 32 RPM rows x 20 throttle cols, with real
  axes (RPM fe[8], throttle fe[27]) and the AFR curve (fe[2], 128=lambda 1.00).
  Validated: main-fuel delta is uniformly richer in the aftermarket map.

Viewer now has a "Triumph tables" tab: drop a real .hex (or pick two) and it
decodes, unpacks, and renders the fuel/ignition tables as heatmaps with real
axes, plus an A->B difference view. build_viewer.py embeds the c.a/s.a directory
so any map resolves in-browser.

Catalogue dropdown: download_maps.py fetches map .hex files into maps_cache/;
serve.py serves the viewer over http so the dropdown can fetch them (drag-and-drop
still works on file://).

Proprietary map binaries (*.hex, *.dec.bin, maps_cache/) are gitignored — code and
docs only.

tunie/viewer/serve.py

@@ -0,0 +1,44 @@
+"""Serve the tunie viewer locally so the catalogue dropdown can fetch cached maps.
+
+    python3 serve.py            # serves this folder at http://localhost:8000
+    python3 serve.py 9000       # custom port
+
+Browsers block fetch() on file:// URLs, so the Triumph-tables catalogue dropdown
+only works when the viewer is served over HTTP. Drag-and-drop works either way.
+Run download_maps.py first to populate maps_cache/.
+"""
+
+from __future__ import annotations
+
+import http.server
+import socketserver
+import sys
+import webbrowser
+from pathlib import Path
+
+HERE = Path(__file__).parent
+
+
+def main() -> int:
+    port = int(sys.argv[1]) if len(sys.argv) > 1 else 8000
+    if not (HERE / "maps_cache" / "index.json").exists():
+        print("note: maps_cache/index.json not found — run download_maps.py first "
+              "for the catalogue dropdown (drag-and-drop still works).")
+
+    handler = lambda *a, **k: http.server.SimpleHTTPRequestHandler(*a, directory=str(HERE), **k)
+    with socketserver.TCPServer(("127.0.0.1", port), handler) as httpd:
+        url = f"http://localhost:{port}/tunie-viewer.html"
+        print(f"serving {HERE} at {url}\nCtrl-C to stop.")
+        try:
+            webbrowser.open(url)
+        except Exception:
+            pass
+        try:
+            httpd.serve_forever()
+        except KeyboardInterrupt:
+            print("\nstopped.")
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())