feat(html): warn on the element types the renderer drops, instead of dropping them silently

[andiwand]

🤖 Generated with Claude Code

The render half of #150. translate_element's default: carried a // TODO log and threw the element and its whole subtree away without a word. It now names the type through state.logger(), which WritingState has carried all along.

default:
  ODR_WARNING(state.logger(), "html: dropped unhandled element "
                                  << element_type_name(element.type()));
  break;

element_type_name is a switch over every ElementType enumerator with no default, so a type added to the enum has to be named here too or the build stops.

The corpus test was logging into the void

html_output_test.cpp called the three-argument html::translate, so the renderer got Logger::null() — nothing it has ever reported was read. It now gets a logger, at warning level: at verbose the pre-existing per-pdf-operator and per-svm-action debug lines take the run from 1.7k to 23k lines and bury everything. At warning it is 2.0k lines.

What the corpus says

That is the point of the issue — turning "support X" into a ranking. Over all 281 corpus renders:

type	dropped	files
page_break	312	30

Nothing else. page_break is the renderer's only element-level gap, 240 of them in odt and 72 in doc. Worst offenders: efficiency+style-big+various-1.odt (87), 785KB.doc (64), style+svm-various-1.odt (38).

That is #174, and it now has a number on it.

The only other warning the whole corpus raises is one pdf cross-reference recovery.

Verification

• ./odr_test --gtest_filter='all_test_files/HtmlOutputTests.*' — 281 tests, pass.
• No markup changes, so the reference output needs no regeneration.