fix: reject a bracketed host that is not an IPv6 address

[SashaMIT]

Parse("[:::]:5000/repo") returned a reference and a nil error. [:::::::]:5000/repo and [fd00123123123]:75050/repo did the same. The name regexp accepts any run of hex digits and colons inside the brackets. The grammar calls that an RFC 3986 IPv6 address, and none of those strings is one.

[::1]:5000/repo, [2001:db8::1]:5000/repo, and [2001:db8::]:5000/repo still parse. A port is still any run of digits, which is what the grammar spells out. The domain regexp stays a prefilter. DomainRegexp still matches [fd00123123123]:75050, and Parse now rejects it.

go test -count=1 -run 'TestReferenceParse/\[:::' failed on tip with missing expected error: invalid reference format and passes after the change. go test . passes. golangci-lint v2 reports no issues.

[codecov]

Codecov Report

❌ Patch coverage is 81.81818% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.12%. Comparing base (6ccba5a) to head (3e6189a).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
reference.go	81.81%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #29      +/-   ##
==========================================
- Coverage   84.21%   84.12%   -0.09%     
==========================================
  Files           5        5              
  Lines         304      315      +11     
==========================================
+ Hits          256      265       +9     
- Misses         38       39       +1     
- Partials       10       11       +1     

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