Bind the Title to numbering level 0 so every h1 restarts the count

[PiotrCzapla]

We need includes in our pipeline, and the only reliable place I found to do them is at the markdown level. An included document has to keep its own heading numbers, since it also works as a standalone document.

The idea: keep a counter for every level, the title (h1) too, but never show the h1 number. Then h2 restarts after each h1. Which gives us a way to Include markdown with reset counters for headers.

Fable ideas were:

• using section reset - did not work too well
• follows list approach and attach a new counter to all headers after each reset, and keeping a reset as an option on the header.

The code looks good after the second Fable attempt; most of the diff is tests.
Needs AnswerDotAI/mdhtml#59.

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Claude's summary of the change:

mdhtml's schemes now carry the h1 title as level 0 with an empty lvlText, so the heading abstractNum reads the scheme straight through, one Word level per scheme level. Level 0 binds to the Title style with w:suff="nothing" (an empty number takes no tab), levels 1-5 to Heading1-5. Word's multilevel rule then restarts every level below each h1, with no second numbering instance and no per-paragraph numPr. A reference without a Title style gets a warning, since its h1s cannot restart anything.

Tests: 29 pass. test_h1_is_title checks the Title binding, empty text and no suffix; test_title_restarts_numbering covers two documents in one file. The numbering.xml was verified through LibreOffice: 1. / 1.1. / 1. / 1.1. for decimal, 1. / (a) / 1. / (a) for legal, with REF fields printing each target's own number.