core: migrate to zig 0.16

[simonklee]

Mechanically migrate core to Zig 0.16 without changing behavior, ownership or api.

• Pin Zig 0.16.0
• Port the build graph to 0.16 module-level C/C++ APIs; replace @cImport with Translate-C for
  miniaudio and Yoga.
• Add one process-lifetime std.Io.Threaded.init_single_threaded seam; keep existing std.Thread.spawn for renderer/audio (no Io.async / Evented yet)
• Migrate locks, clocks, writers, allocators, and removed std APIs
• Forward SSH/mosh markers in auto terminal env so remote sessions do not inherit host
  capabilities
• Upgrade to Unicode 17 data from uucode

Out of scope: std.Io.Evented, renderer/FFI redesign, fixing pre-existing issues.

Surface	Result
Production exports	347 unchanged
Linux dynamic export names	Identical (1,760)
TS FFI struct layouts	Byte-identical
Linux glibc floor	Still GLIBC_2.17
Release targets / native paths	Unchanged