Implements the PDF's Package 4 (cubevm/cubecode): the cube as a substrate for
storing and introspecting code + AI artifacts.
- opcode.rs: decode/encode codec for a deterministic, safe bytecode (28 ops:
stack arithmetic/logic/shift, comparisons, jumps, CALL_LINK, RET, SYSCALL,
DUP/DROP). Body is always decoded through this codec before execution, so
the cube never runs code that didn't survive decode.
- cell.rs: CodeCell + Kind (Fn/Kernel/Layer/Checkpoint/Variant/Other). Kind
rides in the record doc_type; the call graph is the cube's linked_records,
so CALL_LINK n executes linked_records[n].
- vm.rs: stack-based interpreter over CubeStore. CALL_LINK follows cube edges;
call depth is bounded (no infinite cube loops); deterministic + no unsafe +
no float, so runs are reproducible (prereq for 'navigate and reconstruct
experiments'). Host syscalls introspect the cube (degree/link-exists/trace).
Design decision (the PDF's interesting tension): it says body may be
'bytecode, machine code, or serialized model weights' but the load-bearing
clause is a DSL/runtime that 'walks cube links to load and dispatch functions'
= addressable code-as-data. On this hardware a foreign-machine-code JIT is
neither safe nor needed, so we built the safe bytecode VM. cubetrace/
cubedbt/cubeai (execution capture + ML over traces) are the next layer and are
excluded here.
Verified: ./check (fmt+tests+clippy -D warnings) green; cubecode: 13 tests
(arith, div-by-zero fault, CALL_LINK edge-follow, bad-link fault, recursion
depth bound, syscall trace/degree). Gate: cde1e62 -> +Package4.
CUBELinux-2
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