Restructure refimpl into go-lang and python subdirectories

Move Go reference implementation to refimpl/go-lang/ and add new
Python reference implementation in refimpl/python/. Update build.sh
with renamed draft and simplified tool paths.

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

refimpl/go-lang/ect/errors.go

@@ -0,0 +1,29 @@
+package ect
+
+import "errors"
+
+// Sentinel errors for programmatic handling and logging.
+var (
+	ErrPayloadRequired     = errors.New("ect: payload and privateKey required")
+	ErrKeyIDRequired       = errors.New("ect: KeyID required")
+	ErrInvalidTyp          = errors.New("ect: invalid typ parameter")
+	ErrProhibitedAlg       = errors.New("ect: prohibited algorithm")
+	ErrMissingKid          = errors.New("ect: missing kid")
+	ErrUnknownKey          = errors.New("ect: unknown key identifier")
+	ErrWITSubjectMismatch  = errors.New("ect: issuer does not match WIT subject")
+	ErrAudienceMismatch    = errors.New("ect: audience does not include verifier")
+	ErrExpired             = errors.New("ect: token expired")
+	ErrIATTooOld           = errors.New("ect: iat too far in the past")
+	ErrIATInFuture         = errors.New("ect: iat in the future")
+	ErrMissingClaims       = errors.New("ect: missing required claims (jti, exec_act, par)")
+	ErrPolPolDecisionPair  = errors.New("ect: pol and pol_decision must both be present when either is set")
+	ErrInvalidPolDecision  = errors.New("ect: invalid pol_decision value")
+	ErrReplay              = errors.New("ect: jti already seen (replay)")
+	ErrResolveKeyRequired  = errors.New("ect: ResolveKey required")
+	ErrExtSize             = errors.New("ect: ext exceeds max size (4096 bytes)")
+	ErrExtDepth            = errors.New("ect: ext exceeds max nesting depth (5)")
+	ErrInvalidJTI          = errors.New("ect: jti must be UUID format")
+	ErrInvalidWID          = errors.New("ect: wid must be UUID format when set")
+	ErrParLength           = errors.New("ect: par exceeds max length")
+	ErrHashFormat          = errors.New("ect: inp_hash/out_hash must be algorithm:base64url (e.g. sha-256:...)")
+)