Large map: explored border as a continuous line along the fog edge

The game drew 2x2 px red dots per visited-sector neighbour (0x6c8580), in
64-tile steps and unrelated to the explored mask. Replaced by lines traced
from the explored cells (2x2 tiles), simplified to 0.75 px, rebuilt while
the map is open when the masks change.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/game/ui/megamap.h

@@ -0,0 +1,37 @@
+#pragma once
+// The large map (M, cUI_Megamap): a scrollable view of the 2048x2816 world
+// map image. docs/hud.md, mods/megamap.cpp.
+
+#include <cstdint>
+
+namespace game {
+
+// Scroll position (map pixel at the view's left / top edge).
+constexpr size_t kMegamapScrollX = 0xc016c;
+constexpr size_t kMegamapScrollY = 0xc0170;
+
+inline int32_t MegamapScrollX(const uint8_t* megamap) {
+    return *reinterpret_cast<const int32_t*>(megamap + kMegamapScrollX);
+}
+inline int32_t MegamapScrollY(const uint8_t* megamap) {
+    return *reinterpret_cast<const int32_t*>(megamap + kMegamapScrollY);
+}
+
+struct Vec3 {
+    float x, y, z;
+};
+
+// cUI_Megamap::WorldToMapPixel (0x6cb130, thiscall (out, world x / y / level),
+// ret 8): the map pixel of a world position (world units, 53.66563 per tile).
+// Linear: the world plane projection 0x622700, then x * 0.007446 + 1018.75,
+// y * 0.014914 - 186.95.
+inline Vec3 WorldToMapPixel(uint8_t* megamap, const Vec3& world) {
+    Vec3 pixel = {};
+    using Fn = void(__fastcall*)(uint8_t*, void*, Vec3*, const Vec3*);
+    reinterpret_cast<Fn>(0x006cb130)(megamap, nullptr, &pixel, &world);
+    return pixel;
+}
+
+constexpr float kWorldUnitsPerTile = 53.66563f;  // 0x8946b0
+
+} // namespace game