Problem: need a buffer to display tasks with concealed IDs,
virtual text due dates, strikethrough for done items, and
auto-indent.
Solution: add buffer module with acwrite scratch buffer, syntax
conceal for /id/ prefixes, right-aligned due date extmarks,
header/done highlighting, indentexpr, and view toggling.
Problem: need to render task lists grouped by category or sorted
by priority with concealed IDs and metadata.
Solution: add category_view and priority_view functions that
produce buffer lines with hidden /id/ prefixes, priority flags,
and line metadata for extmark placement.
Problem: need to extract due dates and categories from task
descriptions typed in the buffer.
Solution: add right-to-left token parser for configurable date
syntax (default 'due:') and cat: metadata keys. Supports
Category: prefix syntax for :Todo add commands.
Problem: need persistent task storage with forward-compatible
schema and unknown field preservation.
Solution: add store module with load/save, add/update/delete,
ID allocation, and UDA round-trip preservation. Data stored at
stdpath('data')/todo/tasks.json with version 1 schema.
Problem: need user-configurable settings with sensible defaults.
Solution: add config module that merges vim.g.todo with defaults
for data_path, default_view, default_category, date_format, and
date_syntax.
