fix(files): handle stale treesitter nodes gracefully

[seflue]

Summary

This PR fixes intermittent "Index out of bounds" errors when treesitter nodes become stale after buffer modifications.

The error occurs when:

1. Buffer is parsed, nodes are cached
2. Buffer content changes (user edits)
3. Cached nodes are accessed via async callback (e.g., vim.schedule)
4. Node positions now point to invalid ranges → crash

Example error:

Error executing vim.schedule lua callback: .../treesitter.lua:195: Index out of bounds
stack traceback:
        [C]: in function 'nvim_buf_get_text'
        .../treesitter.lua:195: in function 'get_node_text'
        .../orgmode/files/headline.lua:545: in function 'get_property'

Related Issues

Changes

• Wrap get_node_text() in pcall to gracefully handle stale nodes
• Return empty string on failure instead of crashing
• Add _parse_tick field to track buffer state at parse time
• Add OrgFile:is_tree_stale() for callers that need explicit staleness checks

Checklist

I confirm that I have:

• Followed the Conventional Commits specification (e.g., feat: add new feature, fix: correct bug, docs: update documentation).
• My PR title also follows the conventional commits specification.
• Updated relevant documentation, if necessary.
• Thoroughly tested my changes.
• Added tests (if applicable) and verified existing tests pass with make test.
• Checked for breaking changes and documented them, if any.

[kristijanhusak]

Thanks for the PR! It seems unfinished though.