Add extended task list states

[dereuromark]

Summary

Extends task list syntax beyond [ ] and [x] to support additional states inspired by Org-mode:

Syntax	State	Use Case
[ ]	pending	Todo item
[x]	done	Completed
[-]	cancelled	No longer needed
[>]	deferred	Postponed for later
[?]	question	Needs clarification

Example

- [ ] Pending task
- [x] Completed task
- [-] Cancelled task
- [>] Deferred task
- [?] Question task

HTML Output

<ul class="task-list">
  <li><input disabled="" type="checkbox"/> Pending task</li>
  <li><input disabled="" type="checkbox" checked=""/> Completed task</li>
  <li class="task-cancelled"><input disabled="" type="checkbox" data-state="cancelled"/> Cancelled task</li>
  <li class="task-deferred"><input disabled="" type="checkbox" data-state="deferred"/> Deferred task</li>
  <li class="task-question"><input disabled="" type="checkbox" data-state="question"/> Question task</li>
</ul>

Styling Example

.task-cancelled { text-decoration: line-through; opacity: 0.6; }
.task-deferred { color: #888; font-style: italic; }
.task-question { color: #f90; }
[data-state="cancelled"]::before { content: "✗ "; }

Backwards Compatible

• Existing [ ] and [x] syntax unchanged
• getChecked() still works (deprecated in favor of getTaskState())
• New getTaskState() returns one of ListItem::STATE_* constants

Acceptance plan

• BC: getChecked() returns correct bool for all states
• Wait for official approval from jdot author

Preview

[codecov]

Codecov Report

❌ Patch coverage is 96.55172% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.30%. Comparing base (ec86f65) to head (36b77a7).

Files with missing lines	Patch %	Lines
src/Node/Block/ListItem.php	87.50%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master       #9      +/-   ##
============================================
+ Coverage     92.28%   92.30%   +0.02%     
- Complexity     1406     1422      +16     
============================================
  Files            53       53              
  Lines          3616     3639      +23     
============================================
+ Hits           3337     3359      +22     
- Misses          279      280       +1     

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[dereuromark]

TBD: Add [/] as half or "somewhere between no and yes" = "partially"?