Adds minColSpanForLabel as a character-count proxy for how many grid
columns a metric's title needs, reworks HudWidgetLayout.defaultFor into
a left-to-right row-packing layout using it, enforces the same minimum
as a floor on manual resize, and splits _HudMetricValue's FittedBox so
the title renders at a fixed reference size while only the value
shrinks/grows to fill the remaining space.
Confirms a real, documented interaction with FB-08's _reflowRow: its
even column split has no awareness of a member's title-driven minimum
and can push one below it. Left unfixed per this ticket's own
Out-of-scope section -- _reflowRow's algorithm is FB-08's, not this
ticket's, to change.
Test count 426 -> 434, all green. flutter analyze unchanged (same 4
pre-existing, unrelated info-level issues).
uhryniuk
7b452b7b81