Downgrade: raise Optim compat floor to 1.12 (ForwardDiff 1 incompatible below)

[ChrisRackauckas-Claude]

The MINIMUM-VERSION (downgrade) job fails at the julia-downgrade-compat resolve step with a bare Unsatisfiable. Reproduced locally on Julia 1.10 (the LTS downgrade floor); the real conflict is:

Unsatisfiable requirements detected for package Optim [429524aa]:
 Optim [429524aa] log:
 ├─restricted to versions 1.7.5 by an explicit requirement, leaving only versions: 1.7.5
 └─restricted by compatibility requirements with ForwardDiff [f6369f11]
   to versions: [0.20.0-1.4.1, 1.12.0-2.1.0] or uninstalled — no versions left
     └─ForwardDiff [f6369f11] restricted to versions 1 (the package floor), leaving only versions: 1.0.0-1.4.1

Why the old floor was impossible

• The package compat pins ForwardDiff = "1", so at the downgrade minimum ForwardDiff is in the 1.x band.
• Optim releases 1.5 - 1.11 declare ForwardDiff = "0.10" (no ForwardDiff 1 support). Only Optim >= 1.12 declares ForwardDiff = ["0.10", "1"].
• The old floor Optim = "1.7.5" therefore cannot co-install with ForwardDiff = 1: Optim 1.7.5 sits in the gap between the two ForwardDiff-compatible Optim bands and is dropped, leaving no Optim version.

Fix

Raise the lower bound: Optim = "1.7.5, 2" -> Optim = "1.12, 2". 1.12 is the smallest Optim release that supports ForwardDiff 1, i.e. the smallest co-installable floor. The upper bound is unchanged; no test logic or tolerances touched.

Resolve verification (local, Julia 1.10 LTS)

Mimicking julia-downgrade-compat --min on the merged test project (all [compat] entries pinned to their lower bounds):

• floor Optim = 1.7.5 -> Unsatisfiable (reproduces CI).
• floor Optim = 1.12.0 -> RESOLVE SUCCEEDED (full add/resolve completed, exit 0).

Resolution only — the full test suite was not run locally; PR CI confirms.

Ignore until reviewed by @ChrisRackauckas.