The SDK reaches internal/datetime through settings validation, and that
package had only a cgo strftime implementation, so statically linked
Tentacle packages could not build. Split the locale-aware cgo formatter
into its own file and add a C-locale fallback for builds without cgo.
The reader itself still uses the cgo implementation.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>