Structured pattern errors with position tracking

[tigerwill90]

This PR replace sentinel errors with a new PatternError type that provides structured, programmatic error handling for invalid route patterns. Each error includes the pattern, a categorized reason, a human-readable hint, and a byte range pointing at the offending segment.

Callers can now use errors.As/errors.AsType[T] to inspect pattern errors programmatically, and PatternError.Error() renders a visual diagnostic with a cursor:

pattern: hostname: syntax: label starts with '-'
      -a.com/
      ^

pattern: path: syntax: consecutive '/'
      /foo//bar
          ^^

pattern: path: syntax: optional wildcard allowed only as suffix
      /foo/*{any}/bar
           ^^^^^^

pattern: path: regexp: error parsing regexp: invalid repeat count: `{5,2}`
      /foo/{a:a{5,2}}
              ^^^^^^

The new implementation is also a bit easier to maintain than the previous one.

[codecov]

Codecov Report

❌ Patch coverage is 99.15014% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
error.go	96.87%	1 Missing ⚠️
tree.go	85.71%	1 Missing ⚠️
txn.go	75.00%	1 Missing ⚠️

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