Strip r# prefix from raw identifiers in generated C++ names

[HarnageaGabriel]

Summary

Fixes #1324.

A raw identifier used in an extern "C++" block purely to dodge a Rust
keyword (e.g. pub fn highlight(r#type: i32);) was being emitted verbatim
into the generated C++, including the r# prefix:

void ffi$cxxbridge1$highlight(::std::int32_t r#type) noexcept { ... }

which is not valid C++.

Root cause

ForeignName::parse (syntax/names.rs) is the single place that turns a
Rust identifier into its C++-facing name — used both for the implicit
default name and for explicit #[cxx_name = ...] attributes — but it
never stripped the r# marker before storing the name.

Fix

Strip a leading r# in ForeignName::parse, mirroring the existing
precedent in QualifiedName::parse_unquoted (syntax/qualified.rs), which
already does this for raw identifiers in namespace segments.

This intentionally stays narrowly scoped to the issue: it doesn't attempt
to detect or rename identifiers that, after stripping r#, would collide
with a C++ reserved keyword (e.g. class, template) — the crate has no
C++ keyword list today, and adding one is out of scope for this fix.

Test plan

• Added test_raw_identifier_param in tests/cxx_gen.rs, asserting
  the generated header/implementation contain no r# and the expected
  C++ signature uses the bare identifier.
• cargo test --test cxx_gen passes (4/4).
• cargo build succeeds for the whole workspace.

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