lode: Reimplementing DVC's core in Go without breaking the format

lode reimplements DVC's hot path in Go with a non-negotiable invariant: byte-identical compatibility with DVC 3.x. Static binary, parallel hashing, state DB that avoids re-hashing. No migration, no lock-in. But pipelines are out of scope and benchmarks have context. Here's why that perimeter is an honest technical decision, not a limitation to hide.