Drop redundant typename (C++20 P0634)

[helly25]

Clears all 33 readability-redundant-typename findings across 14 files.

What

C++20 (P0634R3, "Down with typename!") removed the requirement for typename in contexts where only a type can appear. These are all such contexts — alias declarations, member declarations, and requires-clause parameter lists:

-    using RawFirst = typename RawPair::first_type;
+    using RawFirst = RawPair::first_type;

-  typename Algo::StreamState state_;
+  Algo::StreamState state_;

-concept HasStreaming = requires(typename Algo::StreamState state, std::string_view data) {
+concept HasStreaming = requires(Algo::StreamState state, std::string_view data) {

The keyword stays valid where it was written, so this is a readability change, not a correctness one. The relaxed rules are C++20, which is this project's baseline (--cxxopt=-std=c++20), so the best-effort GCC build is unaffected — its support predates the baseline.

Method

Applied with clang-tidy's own fixes, exported per TU with --export-fixes and merged once via clang-apply-replacements, rather than running --fix in parallel across 81 TUs that share headers.

Unlike misc-const-correctness in #275 — where the auto-fix broke the build three different ways — this check's fixes are safe here. I verified that by reading the entire diff: all 66 changed lines are bare keyword removals, with no other edit of any kind.

Test

• readability-redundant-typename reports zero across the affected files.
• bazel test --config=clang //... — 109/109 pass.
• pre-commit run -a green.