chore: modernize CI and update Go toolchain

- Bump Go from 1.19 to 1.26 and update all dependencies
- Rewrite CI workflow with matrix strategy (Linux, macOS, Windows)
- Update GitHub Actions to current versions (checkout@v4, setup-go@v5)
- Update CodeQL actions from v1 to v3
- Fix cross-platform bug in mock/path.go (path.Join -> filepath.Join)
- Clean up dependabot config (weekly schedule, remove stale ignore)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

vendor/github.com/cloudflare/circl/internal/conv/conv.go

@@ -5,6 +5,8 @@ import (
 	"fmt"
 	"math/big"
 	"strings"
+
+	"golang.org/x/crypto/cryptobyte"
 )
 
 // BytesLe2Hex returns an hexadecimal string of a number stored in a
@@ -138,3 +140,34 @@ func BigInt2Uint64Le(z []uint64, x *big.Int) {
 		z[i] = 0
 	}
 }
+
+// MarshalBinary encodes a value into a byte array in a format readable by UnmarshalBinary.
+func MarshalBinary(v cryptobyte.MarshalingValue) ([]byte, error) {
+	const DefaultSize = 32
+	b := cryptobyte.NewBuilder(make([]byte, 0, DefaultSize))
+	b.AddValue(v)
+	return b.Bytes()
+}
+
+// MarshalBinaryLen encodes a value into an array of n bytes in a format readable by UnmarshalBinary.
+func MarshalBinaryLen(v cryptobyte.MarshalingValue, length uint) ([]byte, error) {
+	b := cryptobyte.NewFixedBuilder(make([]byte, 0, length))
+	b.AddValue(v)
+	return b.Bytes()
+}
+
+// A UnmarshalingValue decodes itself from a cryptobyte.String and advances the pointer.
+// It reports whether the read was successful.
+type UnmarshalingValue interface {
+	Unmarshal(*cryptobyte.String) bool
+}
+
+// UnmarshalBinary recovers a value from a byte array.
+// It returns an error if the read was unsuccessful.
+func UnmarshalBinary(v UnmarshalingValue, data []byte) (err error) {
+	s := cryptobyte.String(data)
+	if data == nil || !v.Unmarshal(&s) || !s.Empty() {
+		err = fmt.Errorf("cannot read %T from input string", v)
+	}
+	return
+}