Refactor tree_builder.rs into module directory with 7 files implementing all
23 insertion modes per WHATWG HTML §13.2.6.4 as an explicit state machine.
Includes active formatting elements list with Noah's Ark clause, scope
checking for all 5 scope types, context-sensitive fragment parsing, and
iterative token reprocessing via ProcessResult enum.
Code review fixes: fix formatting element end tag truncation bug (save
node_id before truncate), replace manual element creation in catch-all
StartTag with insert_element_maybe_self_closing, add skip_next_newline for
pre/listing/textarea per spec, add QuirksMode to Document with basic DOCTYPE
detection, refactor all 23 handlers from Token to &Token to eliminate clone
per dispatch, fix self_closing handling for pre/listing/textarea/plaintext,
improve golden test 276 fixture coverage and foster-parent test assertion.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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