bidi: add paragraph analysis and line reordering

[kaptinlin]

Layout engines need paragraph levels before L1, then reordering after choosing actual line boundaries. Add immutable paragraph analysis and line reordering, reusing the existing solver and preserving logical rune coordinates.

Validation: go test -race ./... and go vet ./...; public visual ordering matches the Unicode conformance fixtures.

[benoitkugler]

Hello,

Could give us a little bit more of context, so that we understand why this addition is needed ?

Also, why the current approach (segmenting -> shaping -> wrapping -> re-ordering) our stack enables does not suit your needs ?

[andydotxyz]

I glanced at this and the pure-addition and number of lines doesn't really make clear the underlying problem.