Configurable map size (64-256, non-square), one portal per 64x64 tiles, guarded treasure caves, glowing dig-mark blocks

- New Game dialog with map width/height (steps of 32), remembered in settings
- map dimensions are per game; renderer chunks, minimap and camera follow them
- content scales with area; grace period grows with the distance to the Hero Gate
- treasure caves with rich gold, guarded by hero squads that hold their post
- marked blocks: translucent yellow box with outline on all edges
- savegame v2: packed run-length encoded tiles (256x256 map ~320 KB), v1 still loads
- tests: savegame round trip on 96x128, 256x256 generation/simulation/save

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

DESIGN.md

@@ -18,6 +18,7 @@ An original dungeon management game (genre: dungeon building & defence) that run
 - Textures: **procedural pixel art** (tile atlas)
 - Units: **pixel sprites as billboards** (always facing the camera)
 - Lighting: **dark + torches** – dark base level; claimed areas and light sources (torches, core, portal, gate) are bright, flickering light
+- **Dig marking**: a marked block glows as a translucent yellow box with a yellow outline around all edges
 - **Tile outlines**: subtle dark lines around every tile (floors, wall tops, vertical wall edges), following the irregular surface; toggle with G (later in the graphics settings)
 - The UI is a 2D layer on top, including a minimap
 
@@ -43,6 +44,13 @@ An original dungeon management game (genre: dungeon building & defence) that run
 - Music changes with **soft crossfades** (2.5 s): title → build → battle when heroes invade → storm music while attacking the gate → jingles
 - Sound effects are **synthesised chip sounds** (`src/sfx.rs`), positioned by distance and stereo panning
 
+## Map size
+- Chosen in the **New Game dialog**: width and height separately, **64–256 tiles** in steps of 32 (remembered in the settings)
+- Content scales with the area: rock, gold veins and caves; the Hero Gate lies on the far side, the **grace period grows with the distance** to the gate
+- **One portal per 64×64 tiles**; every claimed portal brings creatures (more portals = faster arrivals, still limited by the lair)
+- Bigger maps get **guarded treasure caves** (one less than the number of portals): very rich gold veins, defended by a hero squad that stays at its post
+- Savegames store the tiles packed (run-length encoded, base64), so even 256×256 maps stay small (~300 KB); version 1 savegames still load
+
 ## Game content
 - **Digging**: mark earth/gold → Pit Imps dig, bring gold home and claim ground
 - **Rooms**: treasury, lair, mushroom garden, training hall, library