Fix documentation package compatibility

[ChrisRackauckas-Claude]

Important

Ignore this PR until it has been reviewed by @ChrisRackauckas.

Summary

• Keep the documentation environment compatible with the package's current 0.1 version line.
• Add a Core regression test that derives the expected documentation compat from pkgversion(PDEBase).

This is independent of #102. The release PR can change 0.1.31 to 0.1.32 without changing this documentation constraint.

Root cause

On clean upstream/master at 01a7e79, the reusable SciML documentation install command failed before the build:

julia +release --project=docs/ -e 'using Pkg; Pkg.develop(PackageSpec(path=pwd())); Pkg.instantiate()'

ERROR: empty intersection between PDEBase@0.1.31 and project compatibility 0.2

The adjacent commit boundary identifies 97c9de5 as the first bad commit:

• Its parent, 1cf0ae2, declares PDEBase version 0.2.0 in both the package and documentation projects. The exact install command succeeds there.
• 97c9de5 changes the package version to 0.1.31 but leaves the documentation compat at 0.2.

Using PDEBase = "0.1" keeps documentation compatible across patch releases and avoids another edit for the pending 0.1.32 release.

Local verification

• Julia 1.12, GROUP=Core: 88/88 passed
  • allocation 12/12
  • array variables 11/11
  • discrete initialization 18/18
  • documentation project regression 1/1
  • public API 46/46
• Julia 1.10, GROUP=Core: 73/73 passed
  • allocation 12/12
  • array variables 11/11
  • discrete initialization 18/18
  • documentation project regression 1/1
  • public API 31/31
• Julia 1.12, GROUP=QA: 20/20 passed.
• The exact reusable-workflow dependency install succeeds and selects local PDEBase v0.1.31.
• julia +release --project=docs/ --code-coverage=user docs/make.jl completes doctests, document checks, and HTML rendering with exit code 0. Local deployment is skipped because no CI deployment environment is present.
• Runic 1.7.0: whole-repository --check exits 0.

As a control, the same Julia 1.10 Core command passes on clean upstream/master; no additional clean-master test failure was found.