Fix a dropped morpheme ID when a zero-width rule wraps one ending at the same node

[johnml1135]

Quick summary

Copy-only rules now preserve a wrapped affix's morpheme ID when both rules mark the same trailing shape node.
The fix only makes the correct signature reachable (41/100 fresh loads); the rest trace to a separate, unfixed ordering defect (#506).
Only two files change: one reordered marking call and one new HermitCrab regression test.

Where to look

• SynthesisAffixProcessAllomorphRuleSpec.ApplyRhs -- the reordered fallback marking that fixes the drop.
• IdentityRuleSignatureMorphDropTests -- two deterministic controls plus the fenos regression, which retries up to 100 fresh language loads.

Deliberately not included

• Fixing the separate ordering defect tracked in HermitCrab: investigate unstable zero-width morpheme identities across fresh processes #506, which keeps the regression from being deterministic on every attempt.

Validation

• dotnet build Machine.sln -- Build succeeded, 0 Warning(s), 0 Error(s).
• dotnet test tests/SIL.Machine.Morphology.HermitCrab.Tests/SIL.Machine.Morphology.HermitCrab.Tests.csproj --no-build --no-restore -- Passed: 108, Failed: 0, Skipped: 0.
• dotnet csharpier check . -- Checked 720 files, no issues.
• pwsh -NoProfile -File scripts/comment-hygiene.ps1 -BaseRef origin/master -- comment-hygiene: clean (lines added since origin/master (merge base 7109531e)).

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Reading this a year from now

SynthesisAffixProcessAllomorphRuleSpec.ApplyRhs marks morphs on the output shape in two places: a fallback for rules with no new output morphs (truncation and zero-width rules, such as a copy-only CopyFromInput), and a loop that re-marks each input morph the rule wraps. When a zero-width rule's own affix and an inner wrapped rule's affix end at the same trailing shape node, whichever call runs second keeps that node's morpheme ID. The fallback used to run first, so the outer rule's ID could overwrite or precede the wrapped rule's. Moving the fallback after the loop makes the wrapped rule's ID stick and the outer rule's ID sort after it, matching the order the fixture in IdentityRuleSignatureMorphDropTests expects.

Decisions, and why

The fenos test retries up to 100 fresh loads and passes on the first correct signature. Measured: 41/100 correct with the fix, 0/100 with it reverted. Asserting every attempt would be red even with the fix; a single assertion would be flaky. Retry-until-first-success is the one assertion the data supports, and it goes red with the fix reverted.

An earlier comment blamed BidirList's unseeded skip-list levels. The source does not support that: same-range annotations tie-break on Annotation.ListID, a call-order counter. The claim was removed.

Deferred, and what would unblock it

Making every fresh parse deterministic needs the separate ordering defect tracked in #506 fixed first; this PR does not attempt that, since the true source of the per-run variation is not yet identified (it is not BidirList's level randomization, per the reading above).

Prior measurements (superseded)

An earlier revision of this PR body reported "five green repetitions; suite 575 passed, one skipped, zero failed" and a seed experiment claiming "12/12 repeatability versus three unseeded outputs," attributed to forcing a fixed seed on BidirList's Random. Those numbers were not reproduced here and the causal claim they rested on did not hold up against the source; the 41/100 and 0/100 counts above are the current, directly measured replacement.

🤖 Generated with Claude Code

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This change is 

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.07%. Comparing base (7109531) to head (5aa7b39).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #500   +/-   ##
=======================================
  Coverage   74.07%   74.07%           
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  Files         456      456           
  Lines       38164    38164           
  Branches     5228     5228           
=======================================
+ Hits        28270    28271    +1     
  Misses       8734     8734           
+ Partials     1160     1159    -1     

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