positionBy()'s default branch returned this.source.start verbatim, so for
custom syntaxes whose source.start lacks an offset (e.g. postcss-html) it
returned a position without offset, violating the Position type which
declares offset as required. rangeBy()/positionInside() were normalized to
always compute offset via sourceOffset() (#2033, issue #2029) but
positionBy() was left behind. Mirror the rangeBy() pattern.

Co-authored-by: oss69U-prep <prep@local>
Co-authored-by: Claude Opus 4.8 (1M context) <[email protected]>

When calling Node#rangeBy() (and therefore Node#error() and
Node#warn()) with index 0, the resulting range covered the whole node
instead of a single character at offset 0.

The end-of-range branch used a truthy check on opts.index, so an index
of 0 fell through and left the end position at the node's end. This is
the same off-by-zero issue that was previously fixed for endIndex; the
index branches were missed. Switch both index checks to a numeric type
check, matching the existing endIndex handling.

index 0 now produces a single-character range, consistent with every
other index value.

fromJSON() attached child nodes through the container constructor, which
runs them back through append()/normalize(). For root-level children that
re-applies insertion spacing normalization and overwrites each node's own
raws.before with the previous sibling's, so toJSON() -> fromJSON() was not
lossless: stylesheets with non-uniform blank lines between top-level rules
came back with their spacing flattened.

Rehydrate children separately and attach them directly, keeping the raws
exactly as serialized.

The container constructor cloned every real Node passed in `nodes`, so a
freshly created (parent-less) node was adopted as a copy and the caller's
original reference never had its `parent` set — later operating on it threw
`Cannot read properties of undefined`. The clone was added only to keep the
source tree intact when moving nodes between roots, so it's skipped when the
node has no parent. Nodes that already belong to another tree are still
cloned, preserving the existing behavior. Closes #1987.

* Add missing `test.run()`

* Fix `Input#origin()` returning incorrect position

* Refactor test with source-map-js