Repair documentation example setup

[ChrisRackauckas-Claude]

Please ignore this PR until reviewed by @ChrisRackauckas.

Summary

• provide the documented position-domain initialization equations in the connections tutorial
• bind OnePort's extended v and i names explicitly in the custom-component tutorial and prose
• add OrdinaryDiffEqSDIRK as the direct docs dependency that owns ImplicitEuler

Stack

This draft is stacked on #480 at source head eb23f169. Its docs-only commit is 7a131c93, rebased on current upstream d0243bd3. Once #480 merges, this branch can be rebased to leave only that docs commit.

Why

The clean-main documentation build had four primary strict example failures:

1. the position-domain connection example did not initialize body.s;
2. bare @extend OnePort() could not infer the model's extended variable bindings;
3. the ParametrizedInterpolation builder was registered as scalar Real (fixed by Fix callable interpolation builder symtype #480);
4. ImplicitEuler was loaded without a direct dependency on its owner package.

The three changes here are limited to the corresponding example setup and docs environment. No examples are disabled or made warn-only.

Local validation

• final Julia 1.12.6 strict stacked build (--project=docs docs/make.jl): exit 0; reached CrossReferences, CheckDocument, Populate, RenderDocument, HTMLWriter, and inventory generation
• the first final-tree attempt completed all executable examples but GitHub returned HTTP 429 for an existing ColPrac link; the unchanged strict rerun passed linkcheck and exited 0
• direct dependency smoke test: using OrdinaryDiffEqSDIRK: ImplicitEuler; println(ImplicitEuler()) passed
• final compatible full test/sources.jl on Fix callable interpolation builder symtype #480: exit 0; ParametrizedInterpolation 10 passed and 1 pre-existing broken
• repository-wide Runic check on the final rebased tree: exit 0
• git diff --check: exit 0

Independent failures observed

Current upstream contains the Chua and isothermal repairs from #477 and #478. GROUP=Core now progresses through those tests and stops at the untouched one-ULP magnetic-flux equality in test/magnetic.jl:66. That clean-main failure is being handled by a separate focused audit; this PR does not change its test or numerical behavior.

[ChrisRackauckas-Claude]

Validation checkpoint for docs commit 98eb1d05 on top of #480:

• official Julia 1.12.6 docs environment re-instantiated with docs_instantiate_exit=0
• full unmodified docs/make.jl invocation with code coverage completed with docs_build_exit=0
• the build reached CrossReferences, CheckDocument, Populate, RenderDocument, HTMLWriter, and inventory generation; only the expected local deployment-skip warning remained
• repository-wide Runic completed with runic_exit=0

No example block was skipped, disabled, caught, or changed to warn-only.

[ChrisRackauckas-Claude]

Final rebase and validation checkpoint:

• docs head 7a131c93 is stacked on Fix callable interpolation builder symtype #480 head eb23f169, with current upstream d0243bd3
• before the lease-protected update, I refetched both remotes and verified the remote docs branch was still exactly 98eb1d05 with no contributor commits; the exact lease update to 7a131c93 succeeded
• the first final-tree strict build completed all executable examples but received HTTP 429 for the existing ColPrac link
• the unchanged strict rerun passed linkcheck and exited 0 through CrossReferences, CheckDocument, Populate, RenderDocument, HTMLWriter, and inventory generation
• the build-generated untracked docs/src/assets/Project.toml was removed; repository-wide Julia Runic check and git diff --check both exited 0

No example or Documenter failure mode was disabled or made warn-only.

[ChrisRackauckas-Claude]

Dependency checkpoint: this stacked docs branch inherits current clean-main minimum-compatibility failures unrelated to the documentation edits. The corresponding #480 downgrade job resolves ModelingToolkitBase 1.0 and fails in test/continuous.jl; focused draft #482 raises the floor to 1.1 and validates continuous 76/76 before the separately audited motor blocker. If this branch's downgrade job reports the same stack, it will be handled through #482 rather than duplicated here.

[ChrisRackauckas-Claude]

CI documentation passed on the final rebased head: Build and Deploy Documentation completed successfully in 36m32s. This independently corroborates the final local strict build. The remaining red jobs are the already-linked clean-main minimum-compatibility (#482) and magnetic Core failures, not documentation failures.

[ChrisRackauckas-Claude]

All three hosted Core variants stop at the same clean-main magnetic equality (Core/magnetic.jl: 2 pass, 1 fail), unrelated to the stacked documentation edits. That one-ULP regression is isolated in focused draft #484. The substantive hosted check for this PR—strict Documentation—passed in 36m32s.