[STUBGEN] Let a file declare a namespace with a prefix directive

[Seven-Streams]

Summary

A file that starts with

// tvm-ffi-stubgen(prefix): tirx
// tvm-ffi-stubgen(skip): tirx.DispatchContext

owns every object registered directly under tirx (tirx.transform.* is another file). On each run the tool appends an empty object/<key> block for every such object that no file of the run defines yet, parents first, after the file's last block, and fills it as usual; an import-section is added after the prefix line when the file has none. skip leaves one object out, and a skipped object counts as not asked for, so the dependency error from #748 names it if something else needs it. Code outside the blocks is untouched, so a one-line skeleton is a valid starting point. Rust target only: a Python object/ block needs a class around it.

The roll-out happens in memory before stage 3, so the new blocks join declared and --check (#746) reports a file with pending blocks as stale. It runs after --init, which rewrites files on disk; --init honours skip as well. Builtin ffi.* keys are never rolled out. Two prefix lines in one file, or one prefix declared by two files, fail the run (exit 2); an unknown prefix is only a warning.

Changes

• stub/cli.py: _roll_out_prefixes and _new_blocks, called once after --init, returning a failure count; _stage_2 subtracts the run's skip keys.
• stub/consts.py: prefix and skip join the pipeline directive kinds.
• docs/packaging/stubgen.rst: the directive in the reference.
• tests/python/test_stubgen_rust.py: the testing namespace rolled out around an existing block, a skip, and a hand-written tail (--check 1, in place 0, --check 0); the same skeleton under --init (blocks written, skip honoured, --check 0); a prefix declared twice; exact matching, insertion order, and the builtin exclusion with a stubbed registry.

Testing

test_stubgen_rust.py + test_stubgen.py: 109 passed; pre-commit lint clean. Against libtvm_compiler.so, a tirx.rs with the two lines above, one more skip, and thirteen ty-map lines for the ir types rolls out 63 blocks in one run, leaves no super:: reference, keeps the trailing mod tail {}, and passes --check on the next run.