POS-scoped deletion blocks metathesis analysis of other words (ParseWord incomplete) #520

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.)

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.