Zed bundles the markdown grammar's block scanner natively, and its
`serialize()` `memcpy`s the open-block stack into tree-sitter's fixed
1024-byte serialization buffer with no bounds check. Markdown with
roughly 255+ nested blocks overflows that buffer, and because it sits at
the front of `struct TSParser`, the overflow clobbers the adjacent
parse-stack pointer and heap. Debug builds of the tree-sitter runtime
catch this with an assertion, but release builds like Zed's have no
check and silently corrupt parser memory — which is why this surfaced as
wild crashes deep in tree-sitter's parse stack rather than clean
failures. Still open upstream as
tree-sitter-grammars/tree-sitter-markdown#243.

This PR points `tree-sitter-md` at a `zed-industries` fork whose
`serialize()` refuses to write state that doesn't fit (bounded by the
same running counter the header writes advance, so the check can't
drift). The scanner then deserializes to a fresh state, and the
pathologically nested region surfaces as ordinary tree-sitter `ERROR`
nodes — visible, safe degradation for an adversarial input class,
deliberately chosen over the two alternatives: truncating the block
stack would deserialize into a plausible-but-wrong state and produce
silently incorrect trees, and the scanner ABI offers no error channel at
all (`serialize()` returns a length into a fixed buffer; there is no way
to fail a parse). A nesting-depth cap at block-open time would give
fully deterministic semantics, but that's a behavior change across 13
scanner call sites that belongs upstream, not in a hotfix fork.

The pinned branch is upstream's `9a23c1a9` (the revision Zed already
pinned) plus exactly two commits, for easy review: the guard
(zed-industries/tree-sitter-markdown@179422edf8d0ee511bd3875eb8942d16a8f1f277)
and regression tests
(zed-industries/tree-sitter-markdown@b596e737286780d7bfa9fcddceaeeb754574b352).
The deep-nesting test aborts on the unguarded scanner and passes with
the guard; a moderate-nesting test pins that inputs fitting the buffer
still parse cleanly. The same change is also up as
zed-industries/tree-sitter-markdown#1 into the fork's default branch
(`split_parser`), so future pin bumps don't lose it; if upstream fixes
#243, we can drop the fork entirely on the next bump.

Closes FR-115

Release Notes:

- Fixed a potential crash when editing Markdown with deeply nested
blocks