Implementing tighter spec for to_next_k_path on PrefixZipper and EmptyZipper.

[luketpeterson]

Fixes #137

[github-actions]

Bench A/B vs base: done

job log · 2026-09-25 13:14:03 UTC

base e4b167c → head 19cc638, 3 round(s), median of each run averaged; negative is faster

bench	cases	geomean	largest gain	largest loss	>5% faster	>5% slower
shakespeare	7	+0.3%	-0.8% shakespeare/shakespeare_words_val_count	+3.3% shakespeare/shakespeare_sentences_get	0	0
cities	5	-0.4%	-4.8% cities/cities_val_count_act	+3.6% cities/cities_val_count	0	0
sparse_keys	96	-0.4%	-17.4% join_sparse/50	+4.0% sparse_zipper_cursor/200	3	0
binary_keys	77	-1.4%	-8.8% binary_descend_until_max_bytes/2000	+5.2% binary_get/2000	3	1
superdense_keys	104	-0.7%	-4.8% superdense_insert/100	+3.3% superdense_zipper_step_iter/200	0	0
act_paths	46	-0.6%	-4.6% shape_map_to_act_cata/random_long	+1.4% size_map_to_act_cata/50000	0	0
zipper_head_owned	12	+0.7%	-1.4% owned_head_write_creation_cleanup/1	+2.5% borrowed_head_read_creation/1	0	0
product_zipper	4	-0.4%	-1.2% product_zipper/generic_pathmap_pathmap	+0.2% product_zipper/introspecting_pathmap_pathmap	0	0

7 case(s) moved more than 5%

bench	case	base	head	change
sparse_keys	join_sparse/50	748 ns	617 ns	-17.4%
binary_keys	binary_descend_until_max_bytes/2000	158.73 µs	144.77 µs	-8.8%
sparse_keys	join_sparse/100	2.20 µs	2.01 µs	-8.5%
binary_keys	binary_insert/1600	186.20 µs	171.00 µs	-8.2%
sparse_keys	join_sparse/200	5.83 µs	5.50 µs	-5.7%
binary_keys	binary_set_val_at_long_create/256	112 ns	106 ns	-5.4%
binary_keys	binary_get/2000	113.57 µs	119.43 µs	+5.2%

Full tables per bench are in the job log and the bench-out artifact.