Clear readability-identifier-length by renaming, not by configuring

[helly25]

Sixth clang-tidy triage PR. Clears all 39 readability-identifier-length findings. .clang-tidy is untouched.

The plan changed once I looked at the code

We had agreed to widen IgnoredParameterNames with m, on the understanding it was the gtest matcher idiom. It is not. In mbo/testing/matchers.h the m is a map:

template<typename Map> inline auto IsKeyOf(const Map& m);
template<typename Map> std::vector<typename Map::key_type> AllKeys(const Map& m);

and in matchers_test.cc, m and mm are a map and a multimap. Renaming them to map and multi_map is clearer and shorter than teaching the linter to ignore a letter across the whole repository — so no config change is needed, and the check keeps working everywhere else.

The rest, case by case

where	change
diff_benchmark.cc	to → to_pos, j → pos
hash_benchmark.cc	d → dist_idx, b → point
ini_file.{h,cc}	_ → unused (the ignored half of a structured binding)
diff_myers.cc	waived for the file — see below

The hash_benchmark.cc rename is worth a note: the obvious choice, d → dist, collides with const LatencyDist& dist in the same loop and would have silently produced dist = kLatencyDists[dist]. Hence dist_idx.

diff_myers.cc keeps its names because they are the notation of Myers' paper: n/m the sequence lengths, d the edit cost, k the diagonal, x/y the grid coordinates, kf/kr the forward and reverse diagonals, lo/hi the window bounds. Expanding those to three characters would break the correspondence to the paper that makes the algorithm checkable. The waiver is a single file-scoped region with that rationale, rather than 13 individual comments.

Test

• readability-identifier-length reports zero across every affected file.
• The waiver is narrow, not a blanket: 20 findings from other checks still fire in diff_myers.cc.
• bazel test --config=clang //... — 109/109 pass.
• pre-commit run -a green.