fix(viewer): one form for every span, pills lead, bars run through threads, and the gutter folds

[folded]

Four things the reviewer found using the centre gutter on a real run.

1. Smell pills lead the rationale

They rendered at the end of the block. On gcp.py +779..+783 the rationale ran
to row 786 and put its backward-incompatible pill beside an unrelated span's
dot at 787 — read as one thing. Pills now sit at the top of the block, beside
the bar's first row. Smells describe the span; they belong where it begins.

2. Bars run through comment threads

An annotation row inside a span's range had no bar cell, so a comment thread
left a hole (google_group_settings.py +19..+72, thread at 20 — the brief cited
the wrong file). The span pass now gives every row in a span's range a bar
segment at the span's depth, annotation rows included. annotations.ts is
untouched; the cell is a plain extra child its queries never see.

3. One form for every span

Multi-line spans moved to the gutter in 5b; single-line spans didn't follow.
They still used the legacy line_notes form — a full-width note box in the
code column with an arrow from the dot. Real case: config.py +81..+81, a
686-character extra-review note nested inside +76..+81, with the parent in
the gutter and the child pushing every line below it down.

A single-line span is now its dot plus its rationale in the gutter's text
column, joining the waterfall like everything else — here the parent runs from
76, the child collides at 81 and stretches row 80, both readable in the strip.
_attachSpanNote, _buildSpanNoteContent, the .line-note-* / .annot-note
CSS and the arrow are deleted. Every extra-review note is single-line, so all
of them move into the gutter.

Reviewer comments are not spans and are unchanged — threads, editor, replies,
resolved header, all tests pass untouched. The line_note:N legacy check for a
promoted span stays and now hides the dot too, since the comment stands in its
place. One selector in comments.ts follows the DOM change.

4. The gutter folds — g

Collapsed is lineno · bars: text column 0 wide, rationales hidden, bars and
dots still showing where every span is and how they nest. Global, remembered in
localStorage (scr-gutter-fold), default expanded. g toggles; so does clicking
a gutter cell's empty area, with a title hint on every cell. No chevron — the
strip is per-file but built of per-hunk grids, so it has no single top.

Hover on any mark shows the full rationale (native title; Chrome's cap is
~1024 chars, the 686-char case renders whole). Clicking a mark unfolds and
scrolls to the block. Marks are widened to their full 6px column as the hit
target.

Collapsed, no row stretches. The waterfall only stretches to place text, so
the compact view is the undistorted diff — the answer to "blank space beside a
long note" for anyone reading code first. The pass releases its stretches on
collapse and re-applies on expand; takeRecords() still guards it. The .diff
split follows, so both code columns widen equally; the panel follows the same
state; Annotations.reflowAll() is called because a toggle moves the columns
the arrows point into.

Gate

410 vitest (405 → 410), 1051 Python unchanged, pyright 0, ruff clean.
Mutations: pills at the bottom (fails the DOM-order assertion) · annotation rows
skipped by the bar pass · a single-line span rendered code-side (9 fail) ·
collapse skipping the stretch release · toggle not re-running the pass (2).

Decide

"Add as comment" on a span's intent is gone. It only ever existed on the
code-side note form; multi-line spans never had it. Smell pills stay one-click
promotable. If intent promotion is wanted it belongs as a small affordance in
every block's pill row — a follow-up.

The status bar advertised space toggle; no such handler existed. Now reads
keys 1-3 fold · g gutter · ? help.