Remember to send flushes for unfolded data layers

[knoepfel]

Each processed data cell has a corresponding "flush" message that is emitted to indicate that that data cell is complete. Such flush messages are not being emitted for data cells produced by an unfold node, yielding unnecessary caching:

❯ ./phlex/bin/unfold_transform_fold
[2025-12-11 12:21:05.228] [info] Number of worker threads: 12
[2025-12-11 12:21:05.319] [warning] Transform clamp_node has 150 cached stores.
⋮
[2025-12-11 12:21:05.340] [info] Max. RSS: 1008.026 MB

This PR fixes this problem:

❯ ./phlex/bin/unfold_transform_fold 
[2025-12-11 12:57:32.195] [info] Number of worker threads: 12
⋮
[2025-12-11 12:57:32.298] [info] Max. RSS: 164.839 MB

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@            Coverage Diff             @@
##             main     #183      +/-   ##
==========================================
+ Coverage   81.13%   81.15%   +0.01%     
==========================================
  Files         116      116              
  Lines        2083     2085       +2     
  Branches      332      332              
==========================================
+ Hits         1690     1692       +2     
  Misses        255      255              
  Partials      138      138              

Flag	Coverage Δ
unittests	81.15% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
phlex/core/declared_unfold.hpp	94.73% <100.00%> (+0.19%)	⬆️

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