refactor(syntax): move the foundation and syntax packages into internal/syntax

[devin-ai-integration]

What and why

First step of moving every package under internal/ into layer directories, one directory per pull request, bottom-up. This one moves the foundation and syntax packages into internal/syntax/:

internal/core/{source,ast,ast/astcodec,pack,diag,lexer,parser,format}
  → internal/syntax/{source,ast,ast/astcodec,pack,diag,lexer,parser,format}

A mechanical relocation: git mv, import-path rewrite across the root module and tools/, and the path strings that follow (Makefile/CI/scripts, sonar-project.properties, packaging/msys2/PKGBUILD, test path strings, the self-model's goPackage references, the file citations in docs/project/validation-constraints.md). No identifier renames, no behaviour change.

tests/hygiene/layering_test.go now names layers by directory (syntax, semantic, ir, check, exec, translate, doc, workspace, frontend, tooling); the former foundation row folds into syntax. Every existing tolerated and removed edge is kept. tests/hygiene/protoconv_test.go accepts imports from any layer directory below frontend.

The examples digest in docs/project/pilot-differential-baseline.json is re-recorded because the self-model's goPackage strings changed; no other figure in the baseline moved.

docs/internals/architecture.md and the roadmap are updated in a docs pull request once the whole stack has landed.

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 and the func Test count are unchanged from develop.

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]

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