Variable anchorNameRuleConst

Inline rule for parsing [[# name]] named anchor targets.

Triggered by a BLOCK_OPEN ([[) token. The rule looks for the # character followed by mandatory whitespace and then the anchor name.

Parsing steps:

1. Consume [[ and optional leading whitespace
2. Require a # token (HASH or TEXT "#")
3. Require at least one whitespace token after #
4. Collect consecutive valid anchor-name characters as the name
5. Require closing ]]

Fails if the anchor name is empty or if ]] is not found.