perf(map-generator): major CPU and memory optimisations (#3860)

## Description:

Five performance improvements to the map generator, measured on three
maps of increasing size. End-to-end time on `world` improved ~15×, heap
allocations ~19×.

| Map | Before | After | Speedup |
|-----|--------|-------|---------|
| bosphorusstraits (~612K tiles) | 578ms / 594MB | 45ms / 134MB | 13× /
4.4× |
| world (~2M tiles) | 2333ms / 2128MB | 150ms / 553MB | 15× / 3.8× |
| giantworldmap (~8M tiles) | 10701ms / 9300MB | 635ms / 2509MB | 17× /
3.7× |

Changes (one commit each):
- **`--workers` flag**: bounds concurrent map processing to limit peak
memory
- **Flat `[]bool` visited sets**: replaced `map[string]bool` keyed by
`fmt.Sprintf` with flat `[]bool` indexed `x*height+y` — the dominant
cost
- **`neighborCoords` with stack buffer**: eliminates per-call slice
allocation for neighbour lookups
- **`Terrain` struct 24→16 bytes**: field reorder + `uint8` type for
`TerrainType`
- **Nil buffers early**: releases image/terrain arrays as soon as
they're no longer needed
- **BFS mark-visited on push**: each tile enters the queue once instead
of up to 4×, halving queue memory


also fixes a bug (according to Claude):

Here's the bug: createMiniMap downscales by averaging/sampling 2x2
blocks, copying field values across — including Ocean=true from the
parent scale. When a single connected ocean at 1x splits into multiple
disconnected bodies at 4x (because narrow water channels disappear when
you halve resolution), those smaller fragments still carry Ocean=true
from the carryover. The 4x processWater call picks the new largest
fragment and sets it to Ocean=true, but never clears the others — so
multiple disconnected bodies end up flagged as Ocean.

This PR's fix: before the new BFS pass, zero out every Ocean flag, so
only the truly-largest body at the current scale ends up marked.

It's incidental to the perf work but it's a real semantic change — the
on-disk .bin files will differ from main on any map where ocean splits
across downscaling. The PR doesn't mention it, which is why I flagged
it.

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

map-generator/main.go

@@ -107,6 +107,9 @@ var maps = []struct {
 // mapsFlag holds the comma-separated list of map names passed via the --maps command-line argument.
 var mapsFlag string
 
+// workersFlag controls how many maps are processed concurrently, bounding peak memory usage.
+var workersFlag int
+
 // logFlags holds all the flags related to configuring the map-generator logging
 var logFlags LogFlags
 
@@ -248,15 +251,20 @@ func parseMapsFlag() (map[string]bool, error) {
 
 // loadTerrainMaps manages the concurrent generation of all selected maps.
 // It spins up goroutines for each map and aggregates any errors.
+// Concurrency is bounded by --workers to cap peak memory usage.
 func loadTerrainMaps() error {
+	if workersFlag < 1 {
+		return fmt.Errorf("--workers must be >= 1, got %d", workersFlag)
+	}
 	selectedMaps, err := parseMapsFlag()
 	if err != nil {
 		return err
 	}
 	var wg sync.WaitGroup
 	errChan := make(chan error, len(maps))
+	sem := make(chan struct{}, workersFlag)
 
-	// Process maps concurrently
+	// Process maps concurrently, bounded by the semaphore
 	for _, mapItem := range maps {
 		if selectedMaps != nil && !selectedMaps[mapItem.Name] {
 			continue
@@ -265,6 +273,8 @@ func loadTerrainMaps() error {
 		mapItem := mapItem
 		go func() {
 			defer wg.Done()
+			sem <- struct{}{}
+			defer func() { <-sem }()
 			mapLogTag := slog.String("map", mapItem.Name)
 			testLogTag := slog.Bool("isTest", mapItem.IsTest)
 			logger := slog.Default().With(mapLogTag).With(testLogTag)
@@ -293,6 +303,7 @@ func loadTerrainMaps() error {
 // It parses flags and triggers the map generation process.
 func main() {
 	flag.StringVar(&mapsFlag, "maps", "", "optional comma-separated list of maps to process. ex: --maps=world,eastasia,big_plains")
+	flag.IntVar(&workersFlag, "workers", 4, "number of maps to process concurrently. reduce to lower peak memory usage.")
 	flag.StringVar(&logFlags.logLevel, "log-level", "", "Explicitly sets the log level to one of: ALL, DEBUG, INFO (default), WARN, ERROR.")
 	flag.BoolVar(&logFlags.verbose, "verbose", false, "Adds additional logging and prefixes logs with the [mapname].  Alias of log-level=DEBUG.")
 	flag.BoolVar(&logFlags.verbose, "v", false, "-verbose shorthand")