feat(cubelinux-2): Package 1 — cubecoords + cubestore from PDF spec

New code (no recycling from prior build). Implements:
- cubecoords: Czyx 4-axis coordinate (pack/unpack u32), Null-class
  classification (Total / Cube 1-4 / User), TriWord tri-channel 64-bit
  codec (6 ASCII + 4 control bits), HeaderFlags + CubeHeader (derived
  flags). All coding decisions documented inline.
- cubestore: CubeBackend trait, HashBackend (HashMap<u32>), CubeStore with
  dependency-free length-prefixed record codec (header TLV + body).
- 8 unit tests, all passing; cargo test clean (0 warn/err).

Per directive: separate git repo; current /home/CUBELinux kept as working
tool; scoped to AI-OS-excluded PDF vision.

.gitignore

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+/target
+Cargo.lock