[Backport] Move visualize overloads to package extension; bump to v0.21.2

[mtfishman]

Summary

Backport of #367 onto the new release-0.21 branch.

Replaces the in-source visualize(::AbstractITensorNetwork, ...) method (in src/abstractitensornetwork.jl) and the two type-piracy methods on AbstractNamedGraph / AbstractDataGraph (in src/visualize.jl) with a single ITensorNetworksITensorVisualizationBaseExt package extension that overloads visualize(::AbstractITensorNetwork, ...) only. The type-piracy methods migrate upstream:

• NamedGraphs.jl v0.11.1 (#166) adds the AbstractNamedGraph overload via its own NamedGraphsITensorVisualizationBaseExt.
• DataGraphs.jl v0.4.1 (#119) adds the AbstractDataGraph overload via DataGraphsITensorVisualizationBaseExt.

Motivation for backporting

Once NamedGraphs v0.11.1 and DataGraphs v0.4.1 register, any user on legacy ITensorNetworks v0.21.x alongside the new NamedGraphs/DataGraphs would hit method-overwrite warnings on the two type-pirated visualize methods. v0.22 may take a while to ship (its prerelease accumulator is still receiving changes), so we publish a v0.21.2 patch carrying the equivalent fix.

Changes

• Adds ext/ITensorNetworksITensorVisualizationBaseExt/.
• Removes src/visualize.jl (and its include from src/ITensorNetworks.jl).
• Removes the visualize(::AbstractITensorNetwork, ...) block from src/abstractitensornetwork.jl.
• Adds ITensorVisualizationBase to [weakdeps] / [extensions] / [compat].
• Bumps [compat] floors: NamedGraphs = "0.11.1", DataGraphs = "0.4.1".
• Bumps package version 0.21.1 → 0.21.2.

Release-branch workflow

This is the first PR against the new release-0.21 branch, modeled on Julia's release-X.Y convention (simplified — no backports-release-X.Y staging tier, since we expect only 1–2 backports before v0.22 ships).

After merge, registration needs to be triggered manually via a /register <merge-sha> commit comment on the merge commit (the Registrator.yml workflow's push trigger only fires on main/master by default; the issue_comment path uses force=true and works regardless of branch).

Status

CI will be red until NamedGraphs v0.11.1 and DataGraphs v0.4.1 actually land in the registry. Opening the PR now mostly for tracking.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.89%. Comparing base (9a4c4a7) to head (269a7f2).

Additional details and impacted files

@@               Coverage Diff                @@
##           release-0.21     #369      +/-   ##
================================================
+ Coverage         74.65%   74.89%   +0.24%     
================================================
  Files                63       63              
  Lines              3101     3095       -6     
================================================
+ Hits               2315     2318       +3     
+ Misses              786      777       -9     

Flag	Coverage Δ
docs	48.30% <0.00%> (+0.06%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.