feat(ie-html): implement table insertion modes and foster parenting #59

Table-related insertion modes (WHATWG tree construction):
- InTable: table structure enforcement, foster parenting fallback
- InTableText: simplified pass-through to original mode
- InTableBody: tbody/thead/tfoot with implicit tr creation
- InRow: tr with implicit td/th handling
- InCell: td/th with cell closing and scope checks
- InCaption: caption closing on table elements
- InColumnGroup: col/colgroup handling
- InSelect/InSelectInTable: option/optgroup handling
- InTemplate: template content mode switching

Foster parenting:
- foster_parenting flag on TreeBuilder
- appropriate_insertion_location() finds table parent
- insert_element/insert_character use foster location when active
- Text in table context is inserted before the table

Supporting infrastructure:
- reset_insertion_mode() walks stack to determine correct mode
- clear_stack_back_to_table/body/row_context helpers
- has_element_in_table_scope, has_element_in_select_scope
- close_cell helper, template_modes stack

4 new tests: table_basic, table_implicit_tbody, foster_parenting,
select_element. 59 total ie-html tests passing.

Author: thomasnemer