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

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

Patch Review

**Status: Looks good**

This adds Makefile rules to produce `libnova_core.rmeta` so dependent Rust modules can use `--extern nova_core=<path>` for compile-time type resolution.

The build is isolated in `.nova_core_rmeta/` to avoid racing with the main `nova_core.o` compilation on rustc's intermediate file names — this is a real problem since rustc writes intermediate objects with deterministic names. The implementation:

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

The `--emit=obj=...` followed by `rm -f` is necessary because some rustc versions require code generation to produce correct metadata. The discarded `.o` is placed in the private directory so it doesn't collide.

The two-step approach (private build → `if_changed,copy` to stable path) ensures consumers see a stable path and the copy only happens when content changes. The `private __modname`, `part-of-builtin`, and `part-of-module` settings correctly replicate the build context.

The downside is compiling nova-core effectively twice (once for .o, once for .rmeta), but this is acceptable for a temporary workaround.

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