Converter: preserve choice type slicing (open/closed rules, all variants)

[ryukzak]

Type slicing on choice elements ($this type discriminator) — the pattern KBV Basis and US Core use on onset[x]/value[x] — was mangled by the converter:

• a slicing root without an explicit type list was dropped entirely, losing slicing.rules (open/closed) and the discriminator
• each slice re-expanded into its own parent element, so the last slice overwrote choices (e.g. KBV Condition.onset[x] with slices Period/Range/dateTime/Age came out as choices: ["onsetAge"])
• variant elements got self-referential slicing.slices nodes with empty match: {} (type discriminators are skipped by buildMatchForSlice)

Fix: collapseChoiceTypeSlicing() rewrites such slicing into the plain choice representation before stack processing — the sliced element becomes the choice declaration (accumulated choices + preserved slicing metadata), each slice becomes a regular typed variant element (choiceOf set), and slice children are re-parented under their variant.

Before (KBV Condition.onset[x], open slicing with 4 type slices):

"onset": {
  "choices": ["onsetAge"],
  "slicing": { "slices": { "onsetAge": { "match": {}, "schema": { "choices": ["onsetAge"] } } } }
}

After:

"onset": {
  "choices": ["onsetPeriod", "onsetRange", "onsetDateTime", "onsetAge"],
  "slicing": { "discriminator": [{ "type": "type", "path": "$this" }], "rules": "open" }
}

• First commit adds the regression tests (red on main), second commit makes them green.
• Verified downstream against atomic-ehr/codegen main (make all fully green with this build linked): generated KBV profiles lose their false validateExcluded checks for open choice variants, previously-lost us-core-smokingstatus value[x] slice bindings materialize, and genuinely restricted profiles keep their exclusions.
• Non-type slicing (pattern/value discriminators), extensions, and unsliced choices are untouched; slicing with any other discriminator shape keeps the existing behavior.