docs: fix wrong examples and time/space claims

[cameroncuster]

Full audit of every @code example, @time/@space claim, and semantic statement across all 136 library headers. Three parallel sweeps (data structures / math+graphs+convolution / trees+strings+monotonic_stack+dsu), each flagged item then hand-verified against the code before editing. Comment-only change; 0 code lines touched.

Fixes (6 files):

File	Was	Now	Why
data_structures_[l,r)/bit.hpp	walk2 returns min r s.t. sum of [0,r] >= sum	sum of [0,r+1)	closed-interval doc copied into the half-open directory; re-derived from code: walk2 returns r with prefix(r) < sum <= prefix(r+1). The [l,r] twin's doc is already correct for its convention
graphs/min_vertex_cover.hpp	@time O(n + q * log n), @space O(n * \alpha(n))	@time O(n + m), @space O(n)	cover() is one BFS pass over the matching graph; there are no queries and no DSU — the old claims were pasted from elsewhere
strings/.../compare_substrings.hpp	if cmp1<0 ... (x3)	if cmp<0 ...	example variable is cmp, not cmp1
math/partitions.hpp	"partitions of i numbers"	"partitions of i"	p[i] counts integer partitions of i (A000041), not partitions of an i-element set
math/num_distinct_subsequences.hpp	"subsquence"	"subsequence"	typo
convolution/min_plus_convolution...hpp	res[k] = min of (a[i]+b[j])	min of (convex[j]+arbitrary[i])	doc referenced params a,b that don't exist; index order matches the code (convex[j], arbitrary[i], i+j==k)

Flagged by the sweeps but verified correct, so NOT changed:

• dijkstra.hpp O(n + m log m) — correct for lazy-deletion Dijkstra (heap holds up to m entries; O(m log m) = O(m log n))
• mod_division.hpp O(log(y)) — Euclid on (y, mod) is O(log min(y, mod)) <= O(log y)
• linear_lca.hpp / linear_kth_par.hpp O(n + q) — queries are O(1) given O(1) hardware bit ops (bit_floor/bit_width), which is the standard model
• derangements.hpp indexing — dp[1]=0 falls out of zero-init; matches A000166
• lcs_queries.hpp O(nm log m + q log m + q log q) — the q log q term is real (per-row query sorts)
• monotonic_stack/cart tree/manacher/suffix array semantics — all exactly match code and tests

Validation:

• diff is 100% //! comment lines (checked mechanically)
• clang-format-22 --dry-run --Werror --style=file:.config/dev.clang-format passes on all 6 files
• all changed doc lines fit 59 columns
• affected tests compile: bit, bit_walk, hopcroft_karp (min_vertex_cover), min_plus_convolution, num_subsequences, partitions, lcp_query_zfunc (cmp_substrs)

Based on doc-contracts (#236) to avoid conflicts; if #236 merges first this rebases cleanly (no overlapping hunks).