`to_mdast("- ```\n\nx\n", &ParseOptions::default())` returns Ok in 1.0.0
release but produces an mdast tree where the bullet list-item ends at
offset 6 while its fenced-code child ends at offset 7 — breaking the
unist invariant that a parent's position envelops its child's. An
analogous case fires for the ordered-list shape `1. ```\n\nx\n`.

This is data-integrity, not a panic: `to_mdast` returns `Ok`, but
downstream consumers indexing into source via `child.position.end.offset`
will read past `parent.position.end.offset`. Affects language servers,
source-mapped renderers, and any tool relying on positional invariants.

Adds a small `assert_position_envelop` helper that walks the mdast tree
and reports parent/child position-mismatch violations. Reusable for
future position-invariant regression tests.

Cause likely lives in `src/construct/list_item.rs`: the list-item event
pair closes on the blank-line offset before the fenced-code construct
finishes capturing content, so `on_exit_list_item` finalizes the
position before the code child gets its broader span. Cheapest fix:
post-process parent positions in `on_exit_list_item` to envelop child
spans.

Verified to fail against `markdown = "=1.0.0"` in release. Found via
in-tree fuzzing campaign.