fix: offer a picture the column that draws it cleanly

[vyncint]

Closes #97.

--track on a picture passed None where the text path passed a real sweep,
so for the one verdict a reader cannot answer by contributing more, the tool
printed the diagnosis and stopped.

Measured on the plan that reported it — vyncint/contribution-art's
eleven-column heart, live data, --today 2026-09-06:

   Cannot be drawn cleanly — 5 days are brighter than the picture wants,
   and nothing takes a contribution away.
+  --start-week 37 draws it cleanly.

and the issue's own reproduction:

	before	after
picture, week 30 (holed, 14)	suggested=null	suggested=41, 0 holes
picture, week 34 (holed, 5)	suggested=null	suggested=41, 0 holes
picture, week 38 (holed, 4)	suggested=null	suggested=41, 0 holes

The call site

The comment there said a picture "is usually the full width of the year, so
there is no column to move it to, and offering one that does not fit is worse
than offering none." True of the four shipped templates — dragon is 53
columns, wave/pulse/invader are 51 — and wrong about every picture
narrower than the year, which is the shape docs/ART.md and #57 ask
contributors to draw.

The overhang it was guarding against is now measured by the sweep rather
than assumed at the call site: a placement that pushes lit cells out of the
year is not a candidate, so a picture with ink to its edges still gets nothing
— for the reason, instead of by refusing to look. (Blank margins don't count,
so dragon still places; it just never has anywhere better to go.)

Why a canvas needs its own sweep

Text has one shade — a day is inside a letter or it is not. A picture has five,
so a day can be a hole by being too bright for the shade it is drawn at:
level 2 with the year's peak on it is a hole exactly as level 0 with a single
commit is. A [[bool; 7]] cannot express that.

Rather than reimplement the test, best_start_week_of builds each candidate
into a real Plan and asks for its own holes() — the same call Report
makes for the number it publishes. A suggestion that disagreed with the verdict
printed beside it would be worse than no suggestion. It costs one plan per
column, at most 53, in a run that talks to the GitHub API first.

Two things that came out of testing it

A suggestion has to be a column you can still start in. Ranked by holes then
by column, the first answer for the heart was week 11 — clean, correct, and
in March. That year has nine placements costing zero holes, and the leftmost is
always the one furthest in the past; the only way to draw there is --backfill
into days five months gone. Among columns that tie, one that has not begun now
wins, which makes the answer week 37.

Holes still win outright — a past column that draws the picture cleanly beats a
future one that does not, because back-dating is a thing this tool does and
unlighting a day is not. The preference only breaks ties.

Text needed this too: the example in #97 suggests --start-week 1 in August.

A clean column deserves to be described as one.

-`--start-week 37` would leave 0 holes instead of 5.
+`--start-week 37` draws it cleanly.

The holed headline carries it as well, which is where it was most missing —
that line is the Action's headline output and the subject of the issue the
shipped consumer opens, so it is where "this year is lost" was actually being
published:

Heart · 2026 — 1 of 56 bright, 5 holes that cannot be unlit — week 37 draws it

Action

New suggested-start-week and suggested-holes outputs, so a workflow can act
on the answer rather than only print it.

Tests

The CLI test is the one that matters. The defect was a single argument at a
call site — every unit test of the sweep passed while the tool said nothing —
so the only layer that could have caught it is the one a user sees.

• a_holed_picture_is_told_where_it_can_be_drawn — text, markdown and json all
  carry it, the headline carries it, and taking the advice actually works:
  re-running at the suggested column reports holes: 0.
• a_picture_on_track_is_not_told_to_move — nothing to offer, nothing said.
• a_picture_is_offered_the_column_that_draws_it_cleanly
• a_full_width_picture_is_still_offered_nothing
• a_placement_that_has_already_begun_loses_a_tie
• a_cleaner_placement_beats_a_later_one

Breaking

plan::best_start_week takes a today. Library-only; the binaries and the
Action are unaffected. Listed under Changed in the changelog, per the 0.x
rule at the top of the file.