chore(build): add mtconnect_sysml_model as a submodule

[ottobolyos]

Summary

Add mtconnect_sysml_model as a tracked git submodule at build/sysml-model, pinned to tag v2.7. The submodule replaces the prior documented-but-untracked external clone pattern (where contributors had to git clone https://github.com/mtconnect/mtconnect_sysml_model.git to some out-of-tree path and pass --xmi manually to MTConnect.NET-SysML-Import).

Why

The regen pipeline that builds Devices/Components/*.g.cs, Devices/DataItems/*.g.cs, the asset / events / samples / conditions hierarchies, and the organizer lists needs a pinned XMI source so the output is reproducible. A submodule makes the version explicit in-tree and ties commit history of regenerated code to a specific upstream model commit.

What changed

• .gitmodules — new entry registering build/sysml-model against https://github.com/mtconnect/mtconnect_sysml_model.git.
• build/sysml-model — submodule pointer at tag v2.7 (commit 25796ac59).
• build/MTConnect.NET-SysML-Import/README.md — regen recipe rewritten to reference the in-tree submodule path; the legacy git clone … /tmp/mtconnect-sysml recipe is retained as an ad-hoc fallback for one-off regens against an unpinned commit.

Bumping the pinned model version

git -C build/sysml-model fetch --tags
git -C build/sysml-model checkout v2.8     # or later
git add build/sysml-model
git commit -m "chore(build): bump SysML model submodule to v2.8"

Then regen against the new tip and commit the .g.cs changes in a follow-up commit. Older versions (v2.5, v2.6, v2.0) work the same way for backports.

Worktree usage

The commit body includes a longer note on git worktree interactions: in short, the submodule's gitdir lives under the primary repo's .git/modules/build/sysml-model/, so per-worktree checkouts must each run git submodule update --init build/sysml-model once, and multi-version parallel regen can use git -C build/sysml-model worktree add … to spin a sibling working tree without disturbing the primary checkout.

Test plan

• git submodule update --init build/sysml-model populates the path.
• MTConnect.NET-SysML-Import consumes --xmi build/sysml-model/MTConnectSysMLModel.xml cleanly.
• Regen output diff against current *.g.cs is whitespace + four v2.6 / v2.7 components losing the spurious Name back-fill — verified separately on fix/issue-136-137 (PR fix(common): drop default values from Device and Component constructors #148).
• CI clones the submodule on checkout (workflow update lands with the first PR that depends on the submodule path).

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