Fix Test Print targeting for saved printer units

[boingball]

Fixes the Unit1 test-print problem reported in #75.

What changed

• Save the profile currently selected in MintPrint Settings before testing.
• When Unit1–Unit7 is selected, copy that profile into the driver's existing one-job transient override.
• Let DriverOpen consume and delete the override as it already does for the regression suite.
• Keep Unit0 unchanged, so ordinary application printing still uses the explicitly activated printer.
• Update the AmigaGuide and changelog.

Verification

• make check passes (all host-side tests).
• git diff --check passes.
• Amiga cross-build was not run locally because m68k-amigaos-gcc is unavailable in this runner.

This avoids silently sending a page labelled Unit1 to Unit0 while preserving the existing explicit Activate workflow.

[boingball]

Follow-up fix for the reported could not copy Unit1 to Unit0 result:

• activation now selects the first readable saved profile, preferring ENV: and falling back to ENVARC:;
• that same source is copied to both Unit0 locations, so a profile saved only in ENVARC: no longer makes activation fail;
• direct Test Print uses the same fallback lookup as a defensive measure.

CI run 169 (make check) passes.

[boingball]

Bumped the release to MintPRINT 1.3.2 across the GUI version string, README, CHANGELOG and Aminet readme. The selected-unit Test Print and ENV:/ENVARC: activation fallback are documented as corrective-release fixes.

CI run 170 passes (make check).