The rustdoc claimed 'None if the symbol has no data in [from_ms, to_ms]',
implying a bar-level None. Empirically (cycle-0011 fieldtest) None is
file-level: it propagates data-server's own None, returned only when no
archived file overlaps the window (far-future window or unknown symbol).
A window that overlaps a loaded file but holds zero bars returns
Some(M1Columns) with empty columns, not None.
Document the distinction precisely (chosen over collapsing in-coverage-
empty into None): None propagates data-server's one file-level meaning
unchanged -- 'no data source to read from' -- rather than overloading it
with a second aura-level 'source present but empty here' meaning. So
Some/None means 'data source present' vs 'nothing to read from', not
'has bars' vs 'has none'; a consumer testing for an empty window must
check cols.close.is_empty(). Doc-only, no behaviour change; ledger
untouched (the contract is not referenced there).
closes#18
Brummel
ac4a8c68cf