docs: Update gnd-cli-expansion spec with implementation progress

Update the spec to document recent implementation work:

- Remove "Subgraph" from Non-Goals as kind:subgraph is now supported
- Document tuple/struct component name handling in ABI codegen
- Add build output structure section showing template directories
- Document subgraph data source codegen (generates subgraph-{HASH}.ts)
- Note that gnd copies WASM when multiple sources share mapping.ts
- Update test counts: 166 lib + 7 cli_commands + 12 codegen tests

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: lutter