This patch fixes an issue where grid items using `grid-area: <name>`
with no matching named grid area or lines would be incorrectly placed
at position 0.
According to https://www.w3.org/TR/css-grid-1/#line-placement:
1. When a `<custom-ident>` doesn't match any named line, placement
should fall back to the first implicit grid line
2. When the same `<custom-ident>` is given for both `grid-*-start` and
`grid-*-end` (which happens with `grid-area: name`), and both fall
back to the same implicit line, the resulting span is 1
Previously, the fallback values were hardcoded to 0 and 1, which placed
items in track 0. The fix changes the fallback to use
`m_explicit_line_count` (the first implicit line index) for both start
and end identifiers. When both reference the same line (start == end),
the existing "both positioned" logic now handles this by setting span=1
and adjusting end accordingly.
