convert single-call recursive lambdas to unnamed IIFEs

[cameroncuster]

What

Converts 7 single-call recursive this auto&& lambdas from named
lambdas into unnamed immediately-invoked function expressions (IIFEs).

The pattern in each case:

auto dfs = [&](this auto&& dfs, ...) { ... };
dfs(args);

becomes:

[&](this auto&& dfs, ...) { ... }(args);

Files

• library/convolution/min_plus_convolution_convex_and_arbitrary.hpp (dnc)
• library/graphs/euler_path.hpp (dfs)
• library/graphs/strongly_connected_components/offline_incremental_scc.hpp (dnc)
• library/trees/centroid_decomp.hpp (dfs)
• library/trees/edge_cd.hpp (dfs)
• library/trees/shallowest_decomp_tree.hpp (dfs)
• library/trees/uncommon/subtree_isomorphism.hpp (dfs)

For centroid_decomp.hpp, the comma-operator return (return dfs(0), p;)
was cleaned into a discarded IIFE statement followed by return p; —
semantically identical and formats cleanly.

Note

Explicit return types (-> int, -> void) are retained where present.
C++ cannot deduce the return type of a recursive lambda whose recursive
call precedes any return, so the IIFE conversion only removes the
redundant name, not the return type.

Verification

• Repo clang-format dry-run passes on all 7 files.
• All 14 affected .test.cpp files compile cleanly under both g++ and clang
  with the repo's flags.
• Self-contained handmade stress tests (count_paths, edge_cd_small_trees)
  pass their internal asserts and exit 0.

Net: 18 insertions, 25 deletions.