Source spans for directive arguments, and the regions `lex()` blanks

[dvejsada]

Summary

lex() knows where every argument of a directive starts and ends, and which regions it treated as literal text. Its results keep neither, so tools that edit or render directive source re-lex it themselves.

1. Argument spans. Directive has start/end for the whole directive, and decoded positional/params, but no span for each argument as written. ForLegalAI/PactTrack needs them in two places:
   • Renaming an identifier (a section or definition id) in every {{ref:}}/{{term:}} means splicing a new positional value. _positional_end in app/routes/pages.py re-scans the source to find it.
   • Assembly §15.7.2 step 5 keeps a placeholder's note "exactly as written". _argument_spans in app/services/assembly.py is a second argument lexer, copying _lex_arguments/_lex_value.
2. Literal regions. lex() blanks fenced code, code spans and HTML comments in Lexed.view but doesn't return where they are. PactTrack's export_inline._literal_regions/_fenced_code rebuild them by diffing text against view and re-matching backtick runs, to render those regions verbatim.

Proposal

• Directive.positional_span: tuple[int, int] | None and Directive.param_spans: dict[str, tuple[int, int]]. The spans cover the value as written, quotes included, as offsets into the lexed text.
• Lexed.literals: list[tuple[str, int, int]], holding (kind, start, end) with kind being fence, code, comment or escape.
• Export closes_fence publicly next to FENCE_OPEN_RE/fence_end. PactTrack's closing-fence checks duplicate it.

Related: #27 (source positions for sections, blocks and diagnostics) and #28 (comment pairing in lex()).