fix(build): separate preprocessor definitions

[kellen-miller]

Summary

- "POSIX," "__STDC_FORMAT_MACROS",
+ "POSIX",
+ "__STDC_FORMAT_MACROS",

Run the unchanged upstream fix discordjs#212 through this fork's build matrix. Head: ba63e4a347771c25bd654f8e3515bf0012d0be01.

Evidence

• Before: Upstream CI awaits maintainer approval.
  After: This PR triggers the same workflow in the fork without changing the code or CI matrix.
• Local source rebuild, codec test, and macro preprocessing check pass. Fork run: https://github.com/kellen-miller/opus/actions/runs/37804078955 .
• Current results: 41 jobs pass, 2 fail, 1 remains queued for a macOS 26 runner.
• Windows 2025 / Node 20 and 22 fail before compilation because npm's node-gyp cannot detect Visual Studio 18. The unchanged baseline has the same two failures: https://github.com/kellen-miller/opus/actions/runs/37799967933 .
• All Linux jobs, including Alpine/musl, pass. The queued job is macOS 26 / Node 25.

Merge Danger

Door: two-way

Blast Radius: build

This PR exists for CI verification; keep the upstream contribution on its existing branch.