Group snippets into sections in the ^B panel

[MarcelInTO]

Closes #38.

Why

Twenty-seven snippets across four schemas were being told apart by their names alone —
Divider (rule), Plate - own page, Part (division), Appendix (division) — which is
the naming convention doing a grouping job badly. The config already grouped them, in
banner comments (BLOCKS., ILLUSTRATIONS., DIVISIONS., BACK MATTER and FRAMING.)
that the panel could not see.

What

A <snippet> may now sit inside a <section name="…">:

<snippets>
  <snippet name="Footnote"><block kind="footnote"/></snippet>

  <section name="Illustrations">
    <snippet name="Plate - own page">
      <illustration role="plate" print-placement="own-page" src="" alt=""/>
    </snippet>
    <snippet name="Headpiece">
      <illustration role="headpiece" src="" alt=""/>
    </snippet>
  </section>
</snippets>

┌──────────────┤Snippet at <block>├──────────────┐
│ ◉ Below  ○ Above  ○ Child                      │
│                                                │
│ Blocks                                         │   <- vim Title, orange
│   Paragraph                                    │
│   Argument                                     │
│   Footnote                                     │
│ Illustrations                                  │
│   Plate - own page                             │
│   Headpiece                                    │
│                          ⟦ Cancel ⟧  ⟦► OK ◄⟧  │
└────────────────────────────────────────────────┘

• File order throughout. Sections group, they do not sort, so ordering the file still
  orders the panel.
• A heading is a label, not a choice. The selection steps over it in whichever
  direction it was travelling, so ^B-arrow-Enter still lands on something insertable
  and OK can never commit a row that inserts nothing. One hook rather than arrow-key
  bindings, so arrows, Home/End, PageUp/PageDown, the mouse and type-ahead are all covered
  — and so the drill can exercise it, since injected keys reach app-scope bindings only.
• Headings are coloured (vim's Title → orange, the one accent the tree does not
  already spend on a node kind), so a row at column 0 cannot be misread as an unindented
  snippet.
• Nothing changes without sections. Same rows, no indent, no headings — the feature is
  invisible to a config that has not asked for it, the way an absent config draws no group.
• A section written wrong never costs its snippets. Unnamed leaves them ungrouped,
  nested folds them into the parent, both reported alongside the other skips — the rule the
  rest of the loader already follows.

Three things found on the way

The panel has been under-sizing itself. It reserved 6 rows for chrome and spends 10, so
a short config got a two-row list inside a twelve-row dialog. Long configs hid it — they
clamp to the screen and scroll — and sections make short ones the case that matters, since
every heading is another row to fit. The number is pinned in §14b now, so a framework change
to the chrome fails there rather than silently shrinking the list again.

ValueChanging.NewValue is a trap in Terminal.Gui 2.4.17. It is writable and reads
exactly like a redirect hook; ListView ignores it. (Handled does cancel, but that only
pins the selection where it is, so Down onto a heading would do nothing.) Re-setting from
ValueChanged behind a re-entrancy guard is what actually moves it. The first version sat
on the heading and the drill caught it — verified through MoveDown, not assumed.

Type-ahead would have broken silently. Typing a letter jumps to the next entry starting
with it; the stock matcher compares against the row as drawn, so every sectioned snippet
would have stopped answering to its own first letter. That jump is documented nowhere,
which is exactly why it was worth confirming it worked before changing the rows —
measured first (typing l reaches the off-screen Letter (division)), then preserved with
SnippetMatcher.

Verification

• §14b gains 22 checks — the parse, the row mapping, the skipping, the colour read back
  off the draw buffer, the panel height, and the matcher both as a function and as actually
  attached to the live list.
• Ten mutations, each killing its own check and no others. Two checks were rewritten
  after surviving a first pass.
• Drill passes on all four CI fixtures (460 / 447 / 434 / 439 checks).
• A drill probe that throws now fails the run instead of wedging it: the exception used
  to escape the timeout callback, the Esc never fired, and the modal sat there until the
  harness deadline. Cost a mutation sweep to find. The probe also derives its row indices
  from the rows rather than hard-coding them, so a build with the headings mutated away
  goes red instead of indexing off the end.

Not in this change

Splitting Blocks further — Text / Verse / Dividers — would help more than the four
sections the banner comments already imply, but that is a judgement about the vocabulary
rather than about fux, so the conversion stays faithful to what the file already says.

🤖 Generated with Claude Code

https://claude.ai/code/session_0146fhu1MJdAWimomPpjX6WY