fix: apply per-candidate index when traversing arrays/maps by a streamed index

[StressTestor]

Note: this PR was prepared by an AI agent (Claude Code) acting on behalf of @StressTestor, not by the user personally.

what

$o[.] over a streamed context only returned the first match. this fixes it.

echo '["a","b"]' | yq '. as $o | keys[] | $o[.]'
# before: a
# after:  a
#         b

it hits maps the same way: echo '{"x":1,"y":2}' | yq '. as $o | keys[] | $o[.]' returned only 1.

why

the index in $o[.] is a collect expression that yields one index set per incoming candidate (for keys[] streaming 0 then 1, it yields [0] then [1]). traverseArrayOperator evaluated it over the whole context but then used only rhs.MatchingNodes.Front(), the first index set, so every candidate but the first was dropped. the . as $k | $o[.] workaround worked because as splits the stream, so each call sees a single candidate and Front() is enough.

fix

pair each index set with its candidate (pkg/yqlib/operator_traverse_path.go):

• when the LHS has one node per candidate (.[] | .[idx]), traverse each LHS node with its own index set
• when the LHS collapses to a single node (a variable like $o), traverse it against every index set

LHS and RHS are still each evaluated once over the whole context, so positional operators like split_doc keep their document index.

tests

regression scenarios for the array and map forms in operator_traverse_path_test.go. full go test ./... passes, and the existing traverse and split-doc scenarios are unchanged.

Fixes #2593.