Enhance Minefield gameplay flow

[nomadjimbob]

What changed

• fixed Minefield start spawning so players do not get placed into the ground
• replaced ratio-based mine generation with a fixed mine count
• added configurable lives, with 0 meaning unlimited retries
• changed round scoring to fastest completion time per arena and persisted best-time tracking
• added right-click marker blocks with configurable material
• changed reveal behavior to single-cell progression instead of Minesweeper-style flood reveal
• made the field gap-aware so only solid playable cells are used for pathing, resets, and mine placement
• updated Minefield HUD defaults to match the icon and colour conventions used by the other minigames
• kept players at the finish during the ending phase instead of immediately returning them to the start

Why

Minefield still had a few rough edges from the first implementation:

• structural arena changes were awkward to reason about
• the start region spawn could place players inside blocks
• reveal behavior was too generous for the intended game
• the field model assumed a fully solid board
• scoring and HUD behavior were not aligned with the rest of the minigame set

Impact

Arena authors can now:

• configure an exact mine count
• configure player lives
• use marker blocks
• build boards with deliberate gaps and voids
• rely on a safer start spawn and clearer HUD defaults

Players now get a stricter block-by-block Minefield round with time-based competition and configurable retries.

Validation

• ./gradlew :plugin:compileJava :plugin:test --tests dev.stemcraft.minigame.minefield.MinefieldArenaHandlerTest