maintenance: mark gcc-118291 pass-self sites; drop obsolete #define filter in grep check

[cameroncuster]

Two small maintenance items from the post-C++23 review.

1. Mark the gcc-bug sites so the reverts don't get lost

The C++23 migration (#229) converted recursive lambdas to deducing this, but 7 tree headers had to keep the pass-self idiom because their lambdas live inside member functions/constructors and hit gcc bug 118291 (wrong code with deducing-this lambdas in member context, present in gcc 14/15). That decision only existed in commit messages; nothing in the code said why these 7 files differ from the other 15.

Added a one-line marker at each site:

// pass-self, not deducing this: gcc bug 118291
auto dfs = [&](auto&& dfs, int u) -> void {

Files: hld.hpp, lca_rmq.hpp, linear_lca.hpp, tree_lift.hpp, hagerup_kth_par.hpp, ladder_decomposition.hpp, linear_kth_par.hpp. When a fixed gcc reaches CI/judge, grep for 118291 to find everything to re-convert.

2. Drop the obsolete #define filter in the grep check

grep_clangformat_cppcheck.sh carried:

# TODO: remove this define filter if/when we move to -std=c++20
grep --invert-match --fixed-strings "#define" ... | grep ... ".begin()" ...

We're on C++23 and no #define in the tree contains .begin()-style member calls anymore (verified: the filtered and unfiltered greps both come up empty). Removed the filter and the stale TODO; the check is now a single grep.

Validation

• clang-format --dry-run --Werror passes on all 7 changed headers
• The begin/end grep check passes without the filter (no matches in library/ or library_checker_aizu_tests/)
• No functional changes — comments and a test-script simplification only