Initial commit: RegionIO Minecraft server core (26.1.2/protocol 775)

Vanilla-faithful overworld generator (final_density + multi-noise biomes),
full connection lifecycle (status/login/configuration/play), chunk streaming,
creative block editing, and the protocol/nbt/registry infrastructure.

README.md

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+# RegionIO
+
+A Minecraft Java Edition server core written in Go, targeting version
+**26.1.2** (protocol **775**). RegionIO implements the connection lifecycle
+(status → login → configuration → play), chunk streaming, block editing, and a
+vanilla-faithful overworld generator built on the real `noise_router`
+`final_density` tree.
+
+## Status
+
+- **Network**: full handshake/status/login (offline mode)/configuration/play
+  state machine with zlib compression, keep-alive, and chunk streaming.
+- **Registries**: 28 synchronized registries + tags, captured verbatim from the
+  26.1.2 vanilla server and sent during configuration.
+- **World**: in-memory chunk cache with memoized, compression-ready
+  `level_chunk_with_light` frames; paletted block containers; heightmaps.
+- **Generation**: bit-faithful overworld terrain from the embedded datapack
+  (`ImprovedNoise`/`PerlinNoise`/`BlendedNoise`/`NormalNoise` + the density
+  function interpreter), plus multi-noise **biomes** (per-chunk, surface layer)
+  via the vanilla `Climate` finder over the official biome parameter table.
+- **Gameplay**: creative block place/break, hotbar item→block mapping, chat,
+  teleport ack, and a randomized bedrock floor / beach & gravel surface pass.
+
+## Build & run
+
+```
+go build ./...
+go run ./cmd/regionio -seed 12345
+```
+
+The world seed defaults to `0`; override it with the `-seed` flag or the
+`REGIONIO_SEED` environment variable. The server listens on `0.0.0.0:25565`.
+
+## Testing
+
+```
+go test ./...
+```
+
+Parity tests (`internal/world/vanilla_parity_test.go`) compare generated surface
+heights against captures from the official server and skip when no capture is
+present.
+
+## Project layout
+
+```
+cmd/regionio/      entry point (config, listener, graceful shutdown)
+internal/
+  protocol/        wire primitives: VarInt, framing, compression, packet IDs
+  nbt/             NBT encoder/decoder (with modified UTF-8)
+  registry/        embedded synchronized registries + tags
+  world/           chunk model, level_chunk encoder, cache, generators, biomes
+  worldgen/        noise core + density-function interpreter + climate finder
+  network/         per-connection state machine (handler/conn/play/login/...)
+  server/          shared core: config, status response, profiles
+```
+
+## Notes
+
+The vanilla `server.jar` and its unpacked `libraries/`/`versions/` are **not**
+included (obtain them from Mojang). The embedded data under `internal/`
+(registries, biome parameters, the overworld datapack) is derived from vanilla
+reports and is all that is required to build and run.