refactor(exec): move the execution packages into internal/exec

[devin-ai-integration]

What and why

Fifth step of the layer-directory move, on top of the internal/check pull request. Moves the execution packages into internal/exec/:

internal/core/{runtime,solve,smt,analysis (with modelform, enginewire),engines,objref}
  → internal/exec/{runtime,solve,smt,analysis/...,engines,objref}

A mechanical relocation: git mv, import-path rewrite across the root module and tools/, the layering-test keys and pins (analysis → export, enginewire → export, export → runtime, runtime → parser, passes keep their new paths; exec is still not permitted to import check), the relative fixture paths that cmd/sysml, repl, grpc, format and model tests read from runtime's and analysis's testdata, scripts/pgo-profile.sh, sonar-project.properties and the self-model's goPackage references.

Two gates forced small edits outside the code: the analysis-library census compares its recorded command string verbatim, so docs/project/analysis-library-census.json now names ./internal/exec/runtime (no measurement changed; make docs-counts re-rendered the one line in docs/project/spec-compliance.md), and the link checker required the three internal/core/runtime/testdata/... links in docs/guide/06-behavior.md to follow the move. The examples digest in docs/project/pilot-differential-baseline.json is re-recorded because the self-model's goPackage strings changed; no other figure moved.

How it was verified

go build ./..., go vet ./..., gofmt -l . (empty), make lint, make docs-check, the full suite with OPENSYSML_REQUIRE_TRAINING_CORPUS=1 OPENSYSML_REQUIRE_PILOT_CORPORA=1 OPENSYSML_REQUIRE_PILOT_LIBRARY_XMI=1 go test -count=1 ./..., the tools/ module (build, vet, test, doc-counts -check), python3 scripts/changelog.py check. go list ./internal/... | wc -l (59) is unchanged; TestPackageLayering reports no edge from exec into check.

Checklist

• make test and make lint pass locally
• Tests added or updated for the change
• Documentation extended where it already covers the surface (see CONTRIBUTING.md)
• Changelog entry added as changes/unreleased/<slug>.<section>.md, not as an edit to CHANGELOG.md
• baselines regenerated and make docs-counts run if a gate count moved (compliance rows need nothing: the census is counted at docs build)
• No internal work-item labels (waves, slices, F4, K5) in the body, docs, or changelog

[devin-ai-integration]

I'll fix CI failures and address comments from users with write access. I'll skip comments containing "(aside)".

• Disable automatic comment, CI, and merge conflict monitoring

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.