#664 GTK3V2MAIN: cppcheck-deep triage (Phase 11a of quality-plan-gtk3v2main-part2)

[Allen D Byrne]

Phase 11 of the GTK3V2MAIN quality-improvement plan (part 2): dedicated review of the
report-only cppcheck-deep CI job's "needs individual judgment" backlog, as scoped by
Phase 2's earlier sampling pass.

Categories in scope: knownConditionTrueFalse, nullPointerRedundantCheck,
arrayIndexOutOfBoundsCond, redundantAssignInSwitch, invalidscanf, duplicateCondition,
identicalConditionAfterEarlyExit, unsignedLessThanZero, redundantAssignment (~98 findings
total).

Each finding was individually investigated (not blanket-suppressed). Real bugs found and
fixed; redundant-but-harmless conditions simplified; vestigial/never-wired debug toggles
left in place with a suppression comment explaining why. Full breakdown in the commit
messages and PR description.