migec 2.1.0 — the four figures everyone already knows how to read

First non-alpha of the rewrite. The three stages, the eight commands and the on-disk formats have
been stable across a3 and a4; what this release adds is the QC layer catching up with them.

Four figures you already know how to read. They come off new tables rather than out of a new
computation, so each is still redrawable from a committed TSV:

• The barcode rank plot, on Cell Ranger's
  axes, because it is the figure every user of a droplet protocol has seen: barcodes sorted by
  content, log-log, knee where cells stop. Never: the y axis is unique UMIs, never reads — one
  over-amplified molecule would otherwise put an empty droplet high on the curve, which is the
  exact artefact the plot exists to show. The call is drawn on the curve, not described in a
  caption. New table <sample>.cell_rank.tsv.
• The MIG size spectrum, molecules and the reads they account for, against log(1 + size).
  Both series, because they peak in different places the moment a library is over-sequenced: most
  molecules are shallow, most reads are in the deep ones, and a figure with only one of them says
  the opposite of a figure with only the other. log1p, so a molecule seen once has a place on
  the axis.
• The rank/Zipf curve, molecule size against rank on log-log. Never: this is why the new
  <sample>.sizes.tsv is written at exact sizes and not power-of-two bins — four bins make
  four steps, and a straight line cannot be told from a bent one. It costs one row per distinct
  depth, a few thousand on a real library, not one row per molecule.
• Unique UMIs and reads per sample barcode, off checkout.summary.tsv. The multiplexed
  analogue of the same question.

Consensus quality is a box, not a thinned scatter. Emitted quality is discrete and capped at
the RT floor, so at any real depth every molecule sits on one or two integers: a cloud of dots
draws that as a flat line whether the bin holds ten molecules or ten million, and the every 17
that kept the SVG small threw away the tails that were the only thing the cloud could have shown.
assemble now accumulates the exact joint distribution of (depth bin, rounded Phred) — both are
small integers, so it is 61 counters per bin per bucket — and assemble.quality_by_depth.tsv
carries real order statistics over every molecule.

Publication defaults on every panel. Transparent background, so one SVG serves a light README,
a dark README and print. One ink colour (#808080), which reads on both. The legend is inside
the plot box: a key in the margin makes every figure wider than its data and is the first thing a
journal asks you to move. Frame is 760x520 rather than 900x560 wide.

The pipeline figure is page-shaped. rankdir = TB and three rank = same groups pinning each
side tool level with the data it reads; it was a 5:1 strip that filled the README column with air.
Transparent, and minibwa is in the downstream box.

Twenty panels now, from sixteen. docs/formats.rst documents every QC table and its columns.