CI: consolidate pipeline and adopt version-switcher v0.18.0

[coretl]

Fixes #92, and upgrades the docs pipeline to the version-switcher v0.18.0
reusable-workflow model (from the publish.yml@v0.7.0-called-directly model on
main) — the same piece of work.

What changed

• uv / pyproject dev env (Add uv-managed pyproject.toml + consolidate CI into ci.yml #92) — add pyproject.toml + uv.lock; lint python/ +
  tests/ with ruff via make lint (ruff fixes applied across the harness).
• CI consolidation (Add uv-managed pyproject.toml + consolidate CI into ci.yml #92) — single ci.yml orchestrating reusable _lint.yml and
  _test.yml, replacing the separate code.yml / tests.yml.
• version-switcher v0.18.0 — consume docs.yml and release.yml directly at
  @v0.18.0, and publish.yml via a new local publish-dispatch.yml shim. This
  replaces the direct publish.yml@v0.7.0 call and the preview-fork.yml dispatcher:
  the shim is the dispatchable file the tag re-dispatch re-fires (so publish now
  needs actions: write), and publish.yml owns the deploy / tag-re-dispatch /
  fork-warn branching (no fork guard in ci.yml).
• docs/myst.yml — bump the plugin to v0.18.0; drop logo_text (logo only).

Notes

• Per Add uv-managed pyproject.toml + consolidate CI into ci.yml #92 the work was to branch off docs; since docs→main already merged (Docs → main on shared version-switcher CI (v0.7.0) #93),
  this targets main instead — same goal.
• pytest / [tool.pytest.ini_options] from Add uv-managed pyproject.toml + consolidate CI into ci.yml #92's prose are intentionally omitted:
  the server's tests run via make -C tests (a socket regression harness), not pytest.
• main is already durable in the deployed site (_sources/main.zip live, Pages on
  GitHub Actions), so guard-default-branch is left at its default (true).
• Opening this PR ran an internal-PR publish that reconstructed the whole site and
  served the pr-94 preview; merging redeploys from main.

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