#665 GTK3V2MAIN: clang-tidy triage (Phase 11b of quality-plan-gtk3v2main-part2)

[Allen D Byrne]

Phase 11b of the GTK3V2MAIN quality-improvement plan (part 2): dedicated review of the
report-only clang-tidy CI job's "needs individual judgment" backlog, as scoped by Phase 2's
earlier sampling pass. Companion ticket to #664 (cppcheck-deep triage), same treatment.

Categories in scope: bugprone-not-null-terminated-result, bugprone-suspicious-string-compare,
bugprone-signed-char-misuse, bugprone-too-small-loop-variable,
bugprone-implicit-widening-of-multiplication-result, bugprone-macro-parentheses (~54 findings
total, all real bug classes per the plan: buffer/comparison/overflow/precedence).

Each finding individually investigated. Real bugs fixed; mechanical/cosmetic instances
(macro-parentheses, signed-char casts) fixed directly since they're low-risk syntax-only
changes. Full breakdown in the commit message and PR description.