Claude review: gpu: nova-core: emit Rust metadata for dependent modules

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

Generates `.rmeta` (Rust crate metadata) so `nova-drm` can resolve types at compile time.

```makefile
quiet_cmd_rustc_rmeta_rs = RUSTC RMETA $@
      cmd_rustc_rmeta_rs = mkdir -p $(dir $@); \
        $(rust_common_cmd) --emit=metadata=$@ \
        --emit=obj=$(basename $@).o $<; \
        rm -f $(basename $@).o
```

Key design decisions:
- Builds in a private directory (`.nova_core_rmeta/`) to avoid racing with the main `nova_core.o` build during parallel make. This is a sensible precaution.
- The intermediate `.o` file from the metadata build is immediately deleted since only the `.rmeta` is needed.
- A final `if_changed,copy` step places the metadata at a stable path.

**Minor observation:** The `--emit=obj=$(basename $@).o` plus `rm -f` is needed because rustc insists on emitting an object when also emitting metadata. The `rm -f` cleanup is fine.

**No blocking issues.**

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Generated by Claude Code Patch Reviewer