Fix unlabeled tool steps getting dangling numeric source refs after roundtrip

[jmchilton]

Summary

• Extend _unlabeled_input_ sentinel pattern to all unlabeled step types (tool, pause, subworkflow, pick_value)
• Export assigns _unlabeled_step_N sentinels in label_map and on step dicts so source references remain stable across round-trip
• Converter strips sentinels back to label: None on reimport
• Normalize registers numeric IDs alongside sentinel labels in runs_by_label for anonymous subworkflow output resolution

Closes #146

Test plan

• All 113 tests pass (including abstract export tests)
• Round-trip test for unlabeled tool step verifies connection target and step type are preserved
• Round-trip test for unlabeled pause step verifies same

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 41.03%. Comparing base (024d49f) to head (c117052).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #149   +/-   ##
=======================================
  Coverage   41.03%   41.03%           
=======================================
  Files          51       51           
  Lines        1974     1974           
  Branches      441      441           
=======================================
  Hits          810      810           
  Misses       1047     1047           
  Partials      117      117           

Flag	Coverage Δ
unittests	41.03% <ø> (ø)

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