count_paths: define vl locally instead of relying on kactl NTT typedef

[cameroncuster]

count_paths_rectangle.hpp and count_paths_triangle.hpp use the vl type alias without defining it — it silently leaks from kactl's NumberTheoreticTransform.h (typedef vector<ll> vl;). If kactl ever renames or drops that typedef, these headers stop compiling.

Fix: declare using vl = vector<ll>; in count_paths_rectangle.hpp (triangle includes rectangle, so one definition covers both). This matches the existing pattern in wildcard_pattern_matching.hpp.

Notes:

• The alias legally coexists with kactl's identical typedef — verified that redeclaration compiles clean with -Wall -Wextra
• Both count_paths tests compile; clang-format passes
• Deliberately not promoting vl into kactl_macros.hpp/template.cpp: that would diverge from upstream KACTL's template (which defines only vi/pii/ll), i.e. change what the team hand-types at contest. The remaining ~20 vector<ll> spell-outs are consistent with KACTL convention and left as-is.