make variable naming consistent across library

[cameroncuster]

Purely cosmetic pass to make variable naming consistent across the library — code and doc comments (@code examples, @param docs, READMEs were audited too). No functional or algorithmic changes; opts for the shorter form wherever two conventions coexisted.

Conventions enforced

Concept	Winner	Losers replaced
accumulated result	res	ret, ans, tmp_res
current element/value	cur	curr
visited marker	vis	seen, used
next node/child index	nxt	next (member arrays in tries; also avoids shadowing std::next)
pushed value	val	elem
priority	pri	priority
count of X	cnt_*	count_*, left_count -> cnt_l
position lists	pos / pos_idx	index / index_into_index

Already-consistent names (tin, siz, dp, idx, tl/tr/tm, l/r, u/v/w) were left as the established standard.

Code changes (15 files)

• data_structures_[l,r)/bit.hpp, data_structures_[l,r]/bit.hpp — ret -> res
• data_structures_[l,r)/bit_uncommon/kd_bit.hpp — ans -> res
• data_structures_[l,r)/seg_tree_uncommon/kth_smallest_query.hpp — left_count -> cnt_l
• data_structures_[l,r)/uncommon/deque_op.hpp — elem -> val
• data_structures_[l,r)/uncommon/mode_query.hpp — index/index_into_index -> pos/pos_idx
• data_structures_[l,r)/uncommon/priority_queue_of_updates.hpp — curr -> cur, priority param -> pri (in code, @param, and @code example)
• flow/hungarian.hpp — used -> vis
• graphs/uncommon/enumerate_triangles.hpp — seen -> vis
• math/count_paths/count_paths_rectangle.hpp — ret/res/tmp_res -> res/cur/tmp
• math/num_distinct_subsequences.hpp — curr -> cur
• strings/trie.hpp, strings/binary_trie.hpp — next member -> nxt
• tests/.../binary_trie.test.cpp — updated for the nxt member rename

Doc-comment changes

• graphs/bcc_callback.hpp — @code example: seen/count_edges -> vis/cnt_edges
• dsu/line_tree.hpp — @code example: next -> nxt
• priority_queue_of_updates.hpp — @code/@param: curr_pri/priority -> pri (example was also rewritten upstream in fix doc nits #220; kept the new version with pri)

Full sweep of all @code...@endcode blocks and repo READMEs confirms no remaining occurrences of the replaced names.

Verification

• clang-format --dry-run --Werror passes with dev.clang-format on all changed files
• All 20 affected test files compile clean with g++ -std=c++20 -O2 -Wall -Wextra
• Merged latest dev (includes fix doc nits #220 doc nits and pass recursive lambda self param by reference #222 recursive-lambda change); the one conflict (priority_queue_of_updates.hpp doc example) resolved in favor of dev's rewritten example with the pri naming applied