Add for/endfor iteration and scoped variable resolution to fill_tokens

[ncoop57]

What

Extends the template engine in mdhtml.md with two capabilities:

1. Iteration — {% for x in y %}…{% endfor %} (jinja) and list-valued {{#name}}…{{/name}} (mustache) now repeat their span once per item, binding each item to a frame pushed onto a scope stack.
2. Scoped name resolution — names are now dotted paths resolved innermost-frame-first through the section stack, so variables inside an iterated section see the current item's fields while still falling back to outer (root) frames. Mustache pushes the item's value itself; jinja if pushes nothing (names stay lexical), for pushes {bind: item}.

Why

These are the core missing pieces for real-world templates that repeat over lists (tables, grant tables, contingency lists) while keeping the byte-identical-preservation guarantee of fill_tokens.

Changes

• mdhtml/md.py: rewrites the fill loop to build a node tree, then render via build/walk; adds _resolve for dotted-path scope lookup and standalone for whole-line marker removal.
• mdhtml/jinja.py: _classify now returns a 4-tuple with bind for for statements; docstring updated.
• mdhtml/mustache.py: _classify passes '.' as the bind so list sections iterate; docstring updated.
• tests/test_export.py: replaces the for-unsupported assertion with a while one; adds test_fill_iteration_and_scopes and test_jinja_for covering list iteration, dotted paths, innermost-frame precedence, staged fills, and table contexts.
• updated examples to showcase iteration