feat(tl): add sum method to ancestral_states

[colganwi]

Summary

• Adds a sum method to ancestral_states that computes the sum of leaf values in each node's subtree.
• Uses an iterative bottom-up traversal via reversed(nx.topological_sort(...)) — O(n), no recursion, no intermediate list storage.
• Respects fixed_nodes (fixed internal nodes are treated as leaves).
• Guards against non-numeric dtypes (same as mean).
• Works for both scalar and array keys.

Test plan

• test_ancestral_states_sum — scalar values summed correctly up tree1 and tree2
• test_ancestral_states_sum_array — array (spatial) values summed element-wise
• test_ancestral_states_sum_fixed_nodes — fixed internal nodes are not overwritten

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.95%. Comparing base (4d64ddc) to head (41dfed3).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #54      +/-   ##
==========================================
+ Coverage   93.92%   93.95%   +0.02%     
==========================================
  Files          35       35              
  Lines        2932     2943      +11     
==========================================
+ Hits         2754     2765      +11     
  Misses        178      178              

Files with missing lines	Coverage Δ
src/pycea/tl/ancestral_states.py	97.22% <100.00%> (+0.14%)	⬆️

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