In `@crates/pylib/build.rs`:
- Around line 14-31: The build script currently calls
canonicalize(...).to_str().unwrap(), which can panic on non‑UTF‑8 Windows paths;
change the emission to use a lossy conversion from the canonicalized Path (e.g.,
Path::to_string_lossy on the value returned by std::fs::canonicalize(&lib_path))
and print that into the cargo:rustc-env=win_lib_path value, and if
std::fs::canonicalize(&lib_path) returns an Err emit a visible build warning via
println!("cargo:warning=...") instead of silently skipping so missing
win_lib_path is obvious; update the code paths referencing canonicalized_path
and to_str().unwrap() accordingly.

In `@crates/codegen/src/compile.rs`:
- Around line 6983-6986: The DictMerge path currently uses the first mapping as
the accumulator which can mutate a caller-provided **kwargs dict; change
compile.rs so that when merging multiple mappings (the loop around emit!(self,
Instruction::DictMerge { index: 1 })), you first allocate a fresh empty
accumulator and merge each mapping into that fresh map instead of using the
first input as the target; specifically, before the for-loop that emits
Instruction::DictMerge, emit an instruction to create a new empty dict (so the
accumulator is new) and then merge all incoming `**` groups into that fresh
accumulator (update the code that emits Instruction::DictMerge and the
surrounding logic in the same block to use the new accumulator).