Add a grammar health checker for two unenforced preconditions

[johnml1135]

HermitCrab imposes two requirements on a grammar that nothing currently reports. A grammar author only discovers a violation as a parse that silently returns nothing, which is a hard failure to diagnose from the outside.

Every segment used must be declared in a CharacterDefinitionTable. An undeclared segment makes the parser refuse every word containing it — total, and silent-looking.

Each segment needs a distinct phonological feature bundle within its table. When two segments share one, the parser cannot reliably determine which morphemes are involved.

GrammarHealthChecker.Check(Language) reports both as findings carrying a severity, a stable code, and the offending declarations named so a host can navigate to them.

Design notes

Diagnostic only. It never throws and never changes parse behaviour — a grammar the engine would load still loads. It reports; the host decides what to do.

In the netstandard2.0 engine library, not a tool. So it ships in the SIL.Machine.Morphology.HermitCrab NuGet package and any host — FieldWorks, or anyone testing their own grammar — can call it directly on a loaded Language. Verified by packing: lib/netstandard2.0/SIL.Machine.Morphology.HermitCrab.dll.

The duplicate-bundle check is skipped when a grammar declares no PhonologicalFeatureSystem at all. Such a grammar distinguishes segments by their representation alone, so every bundle is the same empty struct by construction. Reporting that would be a false positive on a correct grammar, and grammars of exactly this shape exist.

Tests

Five tests covering both checks, the clean case, and the feature-less-grammar case. They build the object model directly rather than loading XML, so they have no external fixture dependency. Full HermitCrab suite passes (98/98), zero-warning build, CSharpier and gitlint clean.

Provenance

Rebased onto current master (it was based on 3.9.2). The three files here are also present on the HermitCrab conformance branch (#480), where they were developed further; this branch now carries that newer version, and #480 drops them so the checker lands on its own schedule. Nothing in the conformance suite calls the checker, so the split is clean. Two additional tests live on that branch which run the checker across the real conformance fixtures — they need the fixture tree and the Conformance project, so they stay there rather than move here.

🤖 Generated with Claude Code

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 60.91371% with 77 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.48%. Comparing base (5d26fac) to head (0e80833).

Files with missing lines	Patch %	Lines
...hine.Morphology.HermitCrab/GrammarHealthChecker.cs	60.46%	64 Missing and 4 partials ⚠️
...hine.Morphology.HermitCrab/GrammarHealthFinding.cs	64.00%	6 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #475      +/-   ##
==========================================
- Coverage   73.53%   73.48%   -0.05%     
==========================================
  Files         449      451       +2     
  Lines       37633    37830     +197     
  Branches     5174     5202      +28     
==========================================
+ Hits        27673    27799     +126     
- Misses       8824     8888      +64     
- Partials     1136     1143       +7     

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