Spec conformance: pagination, commit changes, previousCommit, Error shape, select projection, /meta datatypes

[blake-regalia]

Important

Stacked on #28 — merge that first. This branch builds directly on fix/review-findings (its pagination, error-shape, and test scaffolding depend on those fixes), so the PR is based on that branch to keep the diff reviewable: only the seven Phase-3 commits appear here. When #28 merges, GitHub will retarget this PR's base to develop automatically.

Summary

Seven increments closing the highest-impact conformance gaps between this service and the OMG Systems Modeling API spec. The suite grows from 55 to 74 tests, all green against the layer1 + Fuseki test stack.

• Cursor pagination — page[size]/page[after] (the spec's parameter names, previously bound as pageSize/pageAfter and ignored everywhere) now work on every list endpoint, with a Link: rel="next" header advertising continuation:
  • GET .../elements pages at the SPARQL level (ordered by element IRI with a cursor filter, fetching one extra row to detect continuation) instead of always streaming the whole model graph — the endpoint no longer OOMs/times out on large models.
  • projects, branches, tags, commits, queries, and roots page through a shared respondPage helper. Requests without paging parameters keep their historical unpaged responses, including the commits list's newest-first ordering.
• Commit changes endpoints (previously 501) — GET .../commits/{id}/changes and .../changes/{changeId} compute a commit's changes by diffing its model graph against its parent's (layer1 stores the raw SPARQL patch, which cannot be mapped back to per-element DataVersions). Created/modified elements carry their payload at the commit; deleted elements carry a null payload (DataVersion.payload is now nullable per the spec). Change ids derive deterministically from commit + element ids so single changes are stably addressable.
• Commit.previousCommit (previously hardcoded null) — resolved from mms:parent so clients can reconstruct commit history. Flexo's auto-created root commit, which this API hides from the commit list, is never referenced (and is no longer addressable by id); the single-commit GET reads the commit collection since layer1's single-commit response lacks the parent's triples.
• Spec-shaped Error responses — StatusPages returns the spec's Error object instead of plain text; 500s log the cause server-side and return a generic description instead of echoing internal exception messages (IRIs, paths) to clients.
• Query select projection — stored/submitted select was echoed back but ignored at execution; both query-results routes now project each result object to the requested properties.
• Real /meta/datatypes (previously a hardcoded openapi-generator example payload) — serves the 389 schema definitions from the bundled official OMG spec (resources/openapi.json, added to the main resource set): the collection as a $defs map per the spec's response shape, single datatypes by name, 404 for unknown ids.

Deliberately out of scope

ETag/If-Match on success responses, projectUsage/excludeUsed semantics (needs a design decision on used-project federation), and diff/merge remain open — tracked for follow-up work.

Test plan

• docker compose -f src/test/resources/docker-compose.yml up -d
• ./gradlew test — 74 tests, 0 failures (new suites: PreviousCommitTest, ErrorShapeTest, ElementPaginationTest, CollectionPaginationTest, ChangesTest, MetaTest, plus projection cases in QueryResultsTest)

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