Record which walk owns focus_iter_token, so to_next_val stops resuming stale state

[imlvts]

to_next_get_val resumes from focus_iter_token whenever it is not NODE_ITER_INVALID, rather than starting from the focus, and so does to_next_k_path. So the token is not scratch space: it is a promise that an iteration is in progress and positioned where the reader expects. Several movement operations broke that promise, and after each a following to_next_val carried on from the wrong place and reported that nothing was left:

• descend_first_byte (and so to_next_step) stored the token next_items had already advanced past the item it descended into, and NODE_ITER_FINISHED on the branch that did not move.
• descend_first_k_path leaves a token that belongs to the k-path walk, which to_next_k_path legitimately resumes from.
• reset cleared the token only on the branch that popped an ancestor, so a zipper whose whole subtrie lives in one node kept a spent one.
• ascend, ascend_byte, ascend_until and ascend_until_branch restored or left a token that no longer described the focus.

Clearing the token everywhere is the obvious fix and costs ~10% on a to_next_step walk, because to_next_sibling_byte resumes from the same field and to_next_step alternates the two. Instead IterOwner records which walk the token belongs to: the k-path pair claims it, every ascent and descend_first_byte disown it, reset clears it, and to_next_get_val restarts unless the token is its own. The sibling path keeps its fast resume. descend_first_byte stores the token before its child-descent block, which may replace it with the child's own; storing after clobbers it. No API change.

Squashed from five commits on the bugfixes branch, all one defect. Regression test: read_zipper_to_next_val_after_every_movement, nine routes onto the same location followed by a full value walk, plus the reset and four-ascent sequences and a k-path walk. Fails before this change.

lean/FINDINGS.md finding 2 on the lean-fuzzer-restage branch.

[imlvts]

Alternative: store iteration owner as a part of the iteration token, if there's a bit to spare.