Library-first: one root module with a thin core package

[eitamring]

Make ClassMesh importable. The repository becomes one root Go module with a
small core package, and the CLI moves to cmd/classmesh as its first consumer.
Classification behavior does not change.

This took me a week 😓 , lesson learn, no stupid internal package

Summary

• One module, github.com/ClassMesh/classmesh. The workspace and both nested
  modules are removed; go.mod retracts v0.1.0 and v0.1.1, which versioned no
  importable package.
• The root package owns Record, Stage, Classification, and Cascade, and
  imports only the standard library. rules, schema, and stream are public;
  YAML loading and command wiring live under internal.
• The raw diff is 153 files, +2,248/-1,688, almost all of it file moves and
  import rewrites. The genuinely new code is the root package, the extracted
  config loaders, and the proof tests below.
• classmesh version falls back to module build info, so a tagged go install
  reports a real version.

Testing

• A committed 12-case golden suite runs the built binary and compares exit
  codes, stdout, stderr, and created files byte for byte against the
  pre-refactor baseline. Runs in CI.
• A consumer fixture builds against root, rules, and stream from a clean
  cache with GOWORK=off; a negative-compile test pins the internal boundary;
  go list assertions keep the root stdlib-only and yaml/x-text out of the
  public surface.
• Per-record hot paths allocate exactly as before; sinks stay at zero.
  Engine construction pays one extra 32-byte allocation for the cascade's
  copy of its stage list. Interleaved 10x10 benchstat runs show engine and
  pipeline timings unchanged. Three sub-microsecond sink/source benchmarks
  moved 6-14% with the new binary layout; their disassembly and allocations
  are identical, so this is code placement, not a code change.
• make verify, tidy, vet, cgo-check, test, lint, vuln pass locally; CI green.