refactor(spec): major 18's conversions as identifier-sorted entries with an explicit application order, so two retirements merge clean (#20574)

.changeset/20574-major18-conversions-sorted-entries.md

@@ -0,0 +1,21 @@
+---
+'@objectstack/spec': patch
+---
+
+refactor(spec): protocol 18's conversions are authored as identifier-sorted entries with an explicit application order — no value changes (#20574)
+
+Nothing a consumer reads changes. `CONVERSIONS_BY_MAJOR[18]` (46 conversions, same
+order), `ALL_CONVERSIONS` (113, same order) and `MIGRATIONS_BY_MAJOR[18].conversionIds`
+are value-identical to the previous release, so the loader and the migration chain
+apply the same conversions in the same sequence.
+
+What changed is how the list is written, so two major-18 retirements can be in flight
+at once without conflicting in `packages/spec/src/conversions/registry.ts`:
+
+- `CONVERSIONS_BY_MAJOR[18]` is read off `MAJOR_18_CONVERSIONS`: one
+  `{ conversion, order }` entry per conversion, kept sorted by the conversion's
+  identifier and applied by ascending `order` (ties by the conversion's `id`). A
+  retirement adds ONE entry where its identifier sorts — never at the end — with
+  `order` one more than the highest present.
+- A new conversion is defined directly above the definition of the entry that follows
+  it in that list, not at the end of the definitions.