beads: show the stored rows behind an issue on its detail pane

Every field on the detail pane is a reading of a row: the Issue struct names
the columns bd surfaces and drops the rest, humanizeEvent turns two JSON blobs
into a sentence, an Edge keeps an id and a type out of a dependency row. A
reading that went wrong looks exactly like one that did not, so the detail modes
now carry the rows they were built from — the issue's own row first, then the
rows belonging to it in labels, dependencies, comments and events, in the order
the tables were read and the columns came back in. custom_statuses is read for
the lane and is deliberately absent: its rows describe the tracker's statuses,
not this issue. Related tables are capped at RawMax rows and counted whole, so a
busy issue's audit log cannot turn the pane into a second page.

A raw view may not report an absent cell and an empty string as the same thing,
which is the one place cell()'s flattening of a real NULL to "" is wrong.
rowCells.isNull is now the single site that decides, shared by cell and the raw
row, so the two can never disagree; RawCell carries Null beside Value and drops
browse's "NULL" text, which would have put the collision straight back.

The section renders as a collapsed <details> in the stream layout's Past Stand
idiom — it is a check on the rendering, not the reason a reader came. Values are
escaped and deliberately not run through beadLinks.Text: a linkifier rewrites
what it is given, and a value that has been rewritten is no longer the value
that is stored. A cell holding no value renders as a NULL chip, a cell storing
those four characters as the text it stores, an empty one as (empty); a long
value scrolls inside its own cell.