feat: add a modern, drop-in mcpi client package (#61)

[sakebomb]

First slice of epic #61 — a standalone, modern, pip-installable mcpi package so existing Minecraft-Pi curriculum runs unchanged against RaspberryJuice on current Minecraft (Paper 26.2 / Java 25).

Summary

• New mcpi-client/ package reproducing the classic API: from mcpi.minecraft import Minecraft; mc = Minecraft.create(), mc.setBlock/getBlock/getBlocks/getHeight/setSign/postToChat/spawnEntity/..., mc.player.*, mc.entity.*, mc.events.*, plus mcpi.block / mcpi.entity constants, Vec3, Block, Entity.
• Modernized: Python 3.9+, type hints, py.typed, tested. The classic *args + Vec3-flattening ergonomics are preserved (setBlock(x,y,z,id) and setBlock(pos,id) both work); classic numeric block ids still work via the server's legacy-id bridge.
• Wire-correct: command names target the current server registry (world.spawnEntity, events.*, player.events.*), not the stale bundled strings. Its own connection carries over the classic drain() so a stray Fail from an unimplemented command can't desync the next query.
• Naming: distribution is raspberryjuice-mcpi (avoids the legacy PyPI mcpi); the import name stays mcpi, so curriculum imports are unchanged.
• Pi-only commands RaspberryJuice never implemented (checkpoints, camera) are omitted; auth tokens aren't supported here (single-user/classroom-LAN focus — use the raspberryjuice client on a secured server).
• Main README documents both clients; a new mcpi-test CI job runs the suite.

Test plan

• pytest mcpi-client — 25 tests pass on the FakeServer harness: block wire-format (incl. Vec3 args + float-flooring), reply parsing (getBlock/getBlockWithData/getBlocks/getHeight), player pos/tile (ints vs floats), chat, entities (spawn/get/remove), event polls (chat/block/projectile + empty), RequestError on Fail, the drain() stale-reply guard, setSign comma/paren neutralizing, block constants, Vec3 arithmetic, context-manager close
• python -m build --wheel builds raspberryjuice_mcpi-0.1.0-py3-none-any.whl
• Curriculum entry points verified: from mcpi.minecraft import Minecraft, from mcpi import block, entity, block.GOLD_BLOCK.id == 41
• Not yet run against a live Paper 26.2 server — the epic's live canonical-example verification is a tracked follow-up ([Epic] Modern, installable mcpi-compatible Python library (run existing curriculum verbatim) #61); wire format is covered by the FakeServer tests

Follow-ups (tracked in #61)

Live-run canonical Adventures in Minecraft snippets end-to-end; optional Pi-only stubs / token support if a need appears; PyPI publish (with #11).

https://claude.ai/code/session_01AcK3SgXRTVzAKBdPZw72sr