codegen: sibling-module use-imports resolve under cxx-namespace mode (#38)

[shuaimu]

Fixes #38 (the sibling-import half; with #41's crate-root requalification the namespace mode becomes fully coherent).

In flat-export mode a sibling-module use-import emits import sib; and drops the using — correct there, since the sibling's names land at global scope. Under --cxx-namespace/--auto-namespace the sibling's names live in its own namespace, so both emitted reference shapes miss: path-qualified seg::Item (no namespace seg exists in the consumer) and bare single-item imports (use super::varint::VARINT_BUF_LEN → bare VARINT_BUF_LEN).

Fix — at the sibling-import site, additionally emit:

namespace <seg> = ::<sibling::ns>;          // once per sibling module
[export ]using ::<sibling::ns>::<Item>;     // single-item imports

Both land inside the wrapping namespace (not import lines, so the module-import hoist leaves them in place); pub use re-exports keep export.

Real-world case: srpc.wire.serde (mako crates/srpc) — use super::archive::{ReadArchive, WireError, WriteArchive} path shapes and the VARINT_BUF_LEN bare reference. Verification against the full srpc chain is in flight on the merged stack (#41 + this + #42); will report in a comment.

🤖 Generated with Claude Code

https://claude.ai/code/session_01N9DLY5SDRooowERihphNQ1

[shuaimu]

Full-stack verification: with #41 + #42 + #43 + #45 merged locally (verify-stack), the srpc crate (mako crates/srpc) transpiles --crate --auto-namespace with 0 errors and all six emitted modules compile under clang 22 / C++23 modules: srpc.wire.{varint,archive,frame,serde}, srpc.wire, srpc — 6/6. (Issue #44 was sidestepped srpc-side by moving the varint API to idiomatic slice params.)

[shuaimu]

Closing — PRs should not have been opened without explicit instruction. The fix remains on this branch for review whenever wanted.