command: reject empty segments in Parse

[Zahanturel]

command.Parse accepts /crud//create. rs-ucan rejects it with a dedicated EmptySegment error, so the two implementations currently disagree on whether that command is well formed. Parse is what runs on decode (token/delegation/delegation.go:252, token/invocation/invocation.go:290), so the disagreement is about which tokens each library accepts off the wire.

Parse implements the constraints the spec's Segment Structure section states (leading slash, no trailing slash, lowercase) and nothing beyond them, because that section says nothing about a segment being non-empty. ucan-wg/spec#196 proposes adding Segment MUST NOT be empty. to that paragraph, following from ucan-wg/delegation#26. That PR is not merged. rs-ucan made the corresponding change in ucan-wg/rs-ucan#178 while spec#196 was still open, which is the precedent this follows.

The check uses strings.Contains(s, separator+separator) rather than splitting into segments: at that point in the function the leading slash is mandatory and a trailing slash has already been rejected, so an empty segment can only appear as adjacent separators, and / needs no special case. It sits after the lowercase check so the error returned for a given input matches the one rs-ucan returns.

input	before	after
/	ok	ok
/crud/create	ok	ok
/crud//create	ok	ErrEmptySegment
/crud//	ErrDisallowsTrailingSlash	unchanged
/CRUD//create	ErrRequiresLowercase	unchanged

Scope is Parse only; New and Join are untouched. Is their current contract intended? Join short-circuits on zero total length, so New("") returns Top, and TestNew/TestJoin assert that normalizing behaviour, so I have left it alone rather than assume.

This is a behaviour change: Parse gets stricter, so a caller that currently accepts /crud//create will start receiving an error. No fixture or test in the tree carries a command with an empty segment. The new case was confirmed to fail against unmodified Parse before the check was added, in both TestParseCommand/fails_when and TestIsValidCommand/fails_when. go test ./... -tags jwx_es256k passes.