Add automatic linking from grammar term references to their definitions

[lefticus]

Feature Request

Automatically link \\grammarterm{term} references to their \\nontermdef{term} definitions, similar to how section cross-references work.

Example

Current behavior:

• \\grammarterm{statement} → *statement* (just italicized text)

Proposed behavior:

• \\grammarterm{statement} → [*statement*][term.statement] with link to the section containing \\nontermdef{statement}

Benefits

1. Easier navigation through grammar productions - readers can quickly jump to the definition of any grammar term
2. Consistent behavior with existing section cross-reference system
3. Better understanding of grammar structure and relationships

Implementation Considerations

Critical: Namespace Prefix

MUST use term. prefix, NOT gram.

The gram. namespace is already used by existing grammar summary sections:

• gram.general - Grammar Summary General section
• gram.key - Keywords section
• Historically: gram.stmt, gram.expr, gram.dcl, etc.

Using gram. would cause namespace collision and break existing cross-file links.

Technical Requirements

1. Handle malformed patterns gracefully: Must handle \\grammarterm{}{text} (empty first arg) pattern in n3337/n4140 sources (see \\grammarterm{}{text} with empty first argument causes malformed output #78)
2. Use proper Pandoc AST: Use pandoc.Link() elements, not RawInline markdown strings (which don't get re-parsed)
3. Build grammar term index: Similar to LabelIndexer, scan for \\nontermdef{}, \\fmtnontermdef{}, \\renontermdef{} and map terms → sections
4. Generate link reference table: Add entries like [term.statement]: basic.md#stmt.pre at document end
5. Test across all versions: Must work correctly for n3337, n4140, n4659, n4861, n4950, and trunk

Example Index Mapping

translation-unit  → basic.link
statement         → stmt.pre
expression        → expr.comma
class-name        → class.pre

Related Issues

Cross-reference linking issues:

• Broken cross-references with REF: prefix #66 - Broken cross-references with REF: prefix
• Table reference labels missing ID anchors, causing broken links #19 - Table reference labels missing ID anchors
• Cross-file reference anchor mismatches #54 - Cross-file reference anchor mismatches

Priority

ENHANCEMENT - This is a feature request to improve navigation, not a bug fix.

Notes from Failed Implementation Attempt

A previous implementation attempt was reverted due to:

1. Using gram. prefix (namespace collision)
2. Using RawInline markdown (doesn't work with Pandoc's processing model)
3. Not handling malformed \\grammarterm{}{} patterns gracefully
4. Insufficient testing across standard versions

Any future implementation should address these issues.