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/download_maps.py

@@ -0,0 +1,99 @@
+"""Download TuneECU Triumph map files into a local cache for the viewer.
+
+The viewer can render any of these from a dropdown (when served over HTTP, since
+browsers block fetch() on file://). Maps come from the same endpoint the TuneECU
+app uses: https://www.tuneecu.fr/Maps/<path>  (found in the decompile).
+
+    python3 download_maps.py                     # mechanical-odo Bonneville set
+    python3 download_maps.py --filter Bonneville # all Bonneville twin maps
+    python3 download_maps.py 20187 20188 20262   # specific map numbers
+    python3 download_maps.py --all-twins         # every Triumph twin (~big)
+
+Writes maps_cache/<n>Map.hex and maps_cache/index.json (id + description).
+"""
+
+from __future__ import annotations
+
+import argparse
+import json
+import sys
+import urllib.request
+from pathlib import Path
+
+BASE = "https://www.tuneecu.fr/Maps/"
+LIST_URL = BASE + "mapList.dat"
+CACHE = Path(__file__).parent / "maps_cache"
+MAPS_JSON = Path(__file__).parent / "maps.json"
+
+# Default: the 2010 mechanical-odometer Bonneville candidates.
+DEFAULT_IDS = ["20187", "20188", "20191", "20192"]
+
+
+def _get(url: str) -> bytes:
+    req = urllib.request.Request(url, headers={"User-Agent": "tunie"})
+    with urllib.request.urlopen(req, timeout=30) as r:
+        return r.read()
+
+
+def _map_list() -> list[str]:
+    """Return the '<path>/<n>Map.hex' entries from the server's map list."""
+    text = _get(LIST_URL).decode("latin1")
+    return [ln.strip() for ln in text.splitlines() if ln.strip().endswith("Map.hex")]
+
+
+def _descriptions() -> dict[str, list[str]]:
+    if not MAPS_JSON.exists():
+        return {}
+    return {m["id"]: m["description"] for m in json.loads(MAPS_JSON.read_text())}
+
+
+def main() -> int:
+    ap = argparse.ArgumentParser()
+    ap.add_argument("ids", nargs="*", help="specific map numbers to fetch")
+    ap.add_argument("--filter", help="download every map whose path contains this substring")
+    ap.add_argument("--all-twins", action="store_true", help="all Triumph twin maps")
+    args = ap.parse_args()
+
+    entries = _map_list()  # e.g. "Triumph/Bonneville/20187Map.hex"
+    if args.ids:
+        wanted = [e for e in entries if any(f"/{i}Map.hex" in e for i in args.ids)]
+    elif args.all_twins:
+        wanted = [e for e in entries if e.startswith("Triumph/")
+                  and any(k in e for k in ("Bonneville", "Thruxton", "Scrambler",
+                                           "America", "Speedmaster"))]
+    elif args.filter:
+        wanted = [e for e in entries if args.filter.lower() in e.lower()]
+    else:
+        wanted = [e for e in entries if any(f"/{i}Map.hex" in e for i in DEFAULT_IDS)]
+
+    if not wanted:
+        print("Nothing matched.", file=sys.stderr)
+        return 1
+
+    CACHE.mkdir(exist_ok=True)
+    descs = _descriptions()
+    index = []
+    for path in wanted:
+        fname = path.rsplit("/", 1)[-1]           # 20187Map.hex
+        num = fname.replace("Map.hex", "")
+        dest = CACHE / fname
+        if not dest.exists():
+            try:
+                data = _get(BASE + path)
+            except Exception as exc:
+                print(f"  skip {fname}: {exc}", file=sys.stderr)
+                continue
+            dest.write_bytes(data)
+            print(f"  got {fname} ({len(data)} bytes)")
+        index.append({"file": fname, "id": num,
+                      "desc": descs.get(num, [num])})
+
+    index.sort(key=lambda x: x["id"])
+    (CACHE / "index.json").write_text(json.dumps(index, indent=2))
+    print(f"\n{len(index)} maps in {CACHE}/  (index.json written)")
+    print("Serve them with:  python3 serve.py   then use the catalogue dropdown.")
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())