Rename @externalField (parallel-support, deprecation, migration)

Successor name. @computed and @calculated are both candidates. @computed is shorter and matches ComputedField; @calculated reads more clearly when the GraphQL field’s value is derived from but visibly not stored on the row, and avoids overloading "compute" against the generator’s own use of that word in pipeline language. Pick at Spec.

Parallel-support window length. Open between (a) hard expiry tied to a release boundary (e.g. "removed in graphitron 11`"), (b) one full Sikt consumer-migration cycle observed in build logs, (c) open-ended until backlog priority demands it. Sikt has ~49 known `@externalField call sites today (changelog.md R48 entry); the migration tooling decision below feeds back into how short the window can be.

Deprecation-warning channel. The classifier today routes through AUTHOR_ERROR (build-fail) and RejectionKind.DEFERRED (skipped with a reason). Deprecation warnings need a third tier that surfaces in build logs without failing the build. Either: (a) reuse the existing per-site WARN pattern from IdReferenceField’s synthesis shim (`changelog.md R39ish entry calls out the format parentTypeName.fieldName and the canonical replacement string, so future migration tooling can parse the log); (b) introduce an explicit DeprecationWarning carrier on the classified model that the validator surfaces; (c) annotate the classified ComputedField with an optional legacyName and let the validator emit a single aggregated warning. The shim WARN pattern is the cheapest and already proven; (b) and (c) are warranted only if a second deprecation case lands during the parallel window.

Migration tooling shape. Three candidates ordered by cost: (a) document the rename in changelog.md and the getting-started.adoc migration section, no tooling beyond grep; (b) extend graphitron-lsp with a quick-fix code action that swaps the directive name in place (depends on graphitron-lsp being far enough along, see R-id for the LSP plan); (c) a one-shot mvn graphitron-rewrite:migrate mojo that rewrites .graphqls files in place under <schemaInputs>. Sikt’s ~49 sites are tractable by hand; option (a) is likely sufficient unless the parallel window is short enough that automated migration becomes the unblocker rather than the convenience.

Internal model collapse. Both directives map to the same ChildField.ComputedField variant, so the classifier branches on directive name in FieldBuilder and converges to a single carrier immediately. Should the new directive name surface anywhere in the classified model (e.g. as a legacyName slot for the deprecation warning), or stay an artefact of parsing? Tied to the deprecation-channel decision.

Renaming ChildField.ComputedField itself. The variant carries the long-term name; the SDL directive is the user-facing surface that changes.

Wider directive renames (@externalField is the explicit target). If Spec discovers other directive names worth retiring (`@field’s multi-axis overload is a recurring concern but predates this plan), file separately.