POS-scoped deletion blocks metathesis analysis of other words (ParseWord incomplete)

[johnml1135]

A part-of-speech-scoped deletion rule makes metathesis words of another part of speech unparseable. Forward generation still produces them, so ParseWord is incomplete.

Repro

Start from conformance fixture languages/metathesis-phase-isolation (PR #480, 3421588) and add one deletion rule to the surface Morphology stratum, scoped to a part of speech that no metathesis word uses:

<PhonologicalRule id="prHDel">
  <PhoneticInput><PhoneticSequence><SimpleContext naturalClass="ncH" /></PhoneticSequence></PhoneticInput>
  <PhonologicalSubrules>
    <PhonologicalSubrule requiredPartsOfSpeech="posRedup">
      <PhoneticOutput><PhoneticSequence /></PhoneticOutput>
      <Environment>
        <LeftEnvironment><PhoneticTemplate><PhoneticSequence><SimpleContext naturalClass="ncVowel" /></PhoneticSequence></PhoneticTemplate></LeftEnvironment>
        <RightEnvironment><PhoneticTemplate><PhoneticSequence><SimpleContext naturalClass="ncVowel" /></PhoneticSequence></PhoneticTemplate></RightEnvironment>
      </Environment>
    </PhonologicalSubrule>
  </PhonologicalSubrules>
</PhonologicalRule>

(ncH = <SegmentNaturalClass id="ncH"><Segment segment="cH" /></SegmentNaturalClass>; add prHDel to the Morphology stratum's phonologicalRules.)

Word	Root stratum / POS	Expected	Got
nui	SimpleMeta / posSimpleMeta	NIU	none
mu+i	ComplexMeta / posComplexMeta	MI+3SGU	none

Both words still generate forward (they are in the fixture's expected set, and removing prHDel restores them). The rule can never apply to either word in synthesis: wrong part of speech, and their roots live in deeper strata.

What seems to happen

Unapplying the deletion inserts an optional h between every vowel pair, before the part of speech is known, so n u (h) i. The deeper strata's metathesis rules then fail to match u–i adjacency across that optional node, so neither the (h)-present nor the (h)-absent reading reaches the metathesis unapplication. The optional node should be skippable, as it is for rewrite rules.

Notes

• Found while adding reduplication × phonology cases for Prune reduplication matches whose copies cannot agree #519. The fixture now uses V _ a so it doesn't hit this.
• It is independent of Prune reduplication matches whose copies cannot agree #519; master fails the same way.
• Scope unknown: any POS-scoped deletion in a shallower stratum may block metathesis in deeper ones.