Add LogExpFunctions lower-bound compat to fix Downgrade CI

[ChrisRackauckas-Claude]

Note: this PR should be ignored until reviewed by @ChrisRackauckas.

Problem

The Downgrade / Downgrade Tests - Core job has failed on every main run since 2026-06-07 (and on all PRs since, e.g. #1023). Root cause, bisected by registry timeline rather than git history since no LinearSolve commit triggered it:

• ForwardDiff 1.4.0 was registered 2026-06-07 18:03 UTC — exactly between the last green Downgrade run (16:00–17:28) and the first red one (23:44).
• ForwardDiff 1.4.0 is the first version allowing LogExpFunctions 1.x. The downgrade resolver pins direct deps to their lower bounds but maximizes indirect deps, so it now pins LogExpFunctions 1.0.1 into the downgraded manifest (reached via the strong dep PureKLU → ForwardDiff → LogExpFunctions).
• The test sandbox then cannot add the Zygote test dep with allow_reresolve = false: every Zygote 0.7.x requires LogExpFunctions 0.3.x →

  ERROR: Unsatisfiable requirements detected for package Zygote [e88e6eb3]:
  ...restricted by compatibility requirements with LogExpFunctions [2ab3a3ac] to versions: 0.1.0-0.6.40 or uninstalled — no versions left
  

Fix

Add LogExpFunctions to [extras] with compat "0.3". This anchors the downgraded manifest at LogExpFunctions 0.3.x so the Zygote test dep resolves. The normal (non-downgrade) test environment is already held at 0.3.x by Zygote's own compat, so nothing changes there. The bound should be dropped once Zygote supports LogExpFunctions 1.x.

Verification (all run locally on Julia 1.10.11, same procedure as the CI job: julia-downgrade-compat with stdlib skip + mode=deps, then Pkg.test(allow_reresolve=false) with GROUP=Core)

• Unmodified main: reproduces the exact CI failure (Unsatisfiable requirements ... Zygote ... LogExpFunctions).
• With this change, downgraded: manifest pins LogExpFunctions 0.3.29, direct deps stay at their lower bounds, the test sandbox resolves, and the Core test suite launches (Testing Running tests...).
• With this change, normal mode (no downgrade): resolution unaffected, test suite launches.

Caveat on local verification: the sandbox machine reaps long-running Julia processes (external SIGTERM ~10 min in), so the full downgraded Core suite could not be run to completion locally — resolution success and test startup were verified, which is precisely the failure mode of the CI job. The Downgrade CI run on this PR provides the full-suite confirmation.

🤖 Generated with Claude Code

[ChrisRackauckas-Claude]

Independent verification (separate agent, full local run before noticing this PR existed):

Reproduced the failure on unmodified main with the exact CI flow on Julia 1.10.11: ran julia-downgrade-compat's downgrade.jl (--min=@deps --julia=1.10, stdlibs skipped) and then Pkg.test(; julia_args=["--check-bounds=yes"], force_latest_compatible_version=false, allow_reresolve=false) — got the identical Unsatisfiable requirements detected for package Zygote / LogExpFunctions restricted to 1.0.1 error. The downgraded manifest indeed lands LogExpFunctions 1.0.1 via maximized ForwardDiff 1.4.0/DiffRules.

With LogExpFunctions in [extras] + compat capped to 0.3.x (I tested 0.3.29, this PR's 0.3 resolves identically since the resolver maximizes non-minimized packages to 0.3.29), the downgrade resolver produces LogExpFunctions v0.3.29, the test sandbox resolves Zygote v0.7.10, and the full GROUP=Core test suite passes on the downgraded manifest with Julia 1.10.11:

     Testing Running tests...
...
Test Summary: | Pass  Total  Time
Resize        |   70     70  2.3s
     Testing LinearSolve tests passed

Also confirmed no Zygote release supports LogExpFunctions 1.x (Zygote master still has LogExpFunctions = "0.3.29"), so a Zygote-side bound bump is not currently possible.