#666 GTK3V2MAIN: clang-tidy quick closeouts (Phase 11c of quality-plan-gtk3v2main-part2)

[Allen D Byrne]

Phase 11c of the GTK3V2MAIN quality-improvement plan (part 2): quick, low-effort closeouts
from the clang-tidy report-only backlog, as scoped by Phase 2's earlier sampling pass.
Companion ticket to #664 (cppcheck-deep triage) and #665 (clang-tidy triage), same treatment.

Categories: bugprone-casting-through-void (6, sampled as mostly legitimate GTK GObject-cast
idiom), bugprone-reserved-identifier (1, the _USE_MATH_DEFINES pattern).

Each finding individually confirmed as a legitimate, unavoidable idiom (GTK's own type-check
macro, a vendored uthash.h macro, and the Windows CRT's mandated math.h define) and documented
with inline suppression comments rather than altered. Full breakdown in the commit message.