Fix 25 C audit issues (security, safety, memory)

[farkasmark]

Summary

• Fixes 25 issues identified by /c-audit across 16 files (2 critical, 6 high, 10 medium, 7 low)
• All 185 existing tests pass with zero regressions
• 3 issues documented as intentional/safe and skipped (v4l2 write best-effort, font_len parameter, OpenMP lums)

Critical fixes

• sRGB LUT data race — replaced atomic flag with CAS-based 3-state init (uninit → in-progress → ready) to prevent concurrent threads from reading a partially-initialized LUT
• Integer overflow in grid allocation — added size_t casts and INT_MAX bounds check on cols * rows before allocation

High fixes

• Bounds checks on direct ANSI byte writes, fread return validation in tools, snprintf truncation safety via helper function, blip encoder overflow guards, use-after-free on audio stats

Medium fixes

• atoi → strtol with validation in all tools, ftell error checks, malloc NULL checks, division-by-zero guard, negative frame check in WASM player, re-encode return validation, BLIP v1 skip OOB fix, NaN filtering before quickselect

Low fixes

• Updated bench.c to current codecs, blip accessor NULL checks, encapsulated struct access, -Wall -Wextra on emcc builds, removed dead variable

Test plan

• make clean && make compiles without errors
• make test — all 185 tests pass (0 failures)
• No new compiler warnings in src/ or tools/