Build the Tentacle SDK without cgo

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>

internal/datetime/datetime.go

@@ -4,17 +4,7 @@
 
 package datetime
 
-/*
-#include <locale.h>
-#include <stdlib.h>
-#include <time.h>
-*/
-import "C"
-
-import (
-	"time"
-	"unsafe"
-)
+import "time"
 
 type Option struct {
 	Value string
@@ -56,12 +46,6 @@ var dateFormats = []struct {
 	{"abbreviated-ymd", "%Y. %b %-d, %a"},
 }
 
-func init() {
-	locale := C.CString("")
-	defer C.free(unsafe.Pointer(locale))
-	C.setlocale(C.LC_TIME, locale)
-}
-
 func TimeOptions(now time.Time) []Option {
 	return options(timeFormats, now)
 }
@@ -133,22 +117,3 @@ func valid(formats []struct {
 	}
 	return false
 }
-
-func format(now time.Time, spec string) string {
-	cSpec := C.CString(spec)
-	defer C.free(unsafe.Pointer(cSpec))
-	tm := C.struct_tm{
-		tm_year:  C.int(now.Year() - 1900),
-		tm_mon:   C.int(int(now.Month()) - 1),
-		tm_mday:  C.int(now.Day()),
-		tm_hour:  C.int(now.Hour()),
-		tm_min:   C.int(now.Minute()),
-		tm_sec:   C.int(now.Second()),
-		tm_wday:  C.int(int(now.Weekday())),
-		tm_yday:  C.int(now.YearDay() - 1),
-		tm_isdst: -1,
-	}
-	buffer := make([]C.char, 512)
-	length := C.strftime(&buffer[0], C.size_t(len(buffer)), cSpec, &tm)
-	return C.GoStringN(&buffer[0], C.int(length))
-}