Expose BaseDirection through paragraph analysis

[waywardmonkeys]

Parley previously always inferred the paragraph embedding level from the text, even when a caller knew the surrounding reading direction. This is especially wrong for numeric or neutral text and for mixed-direction text whose first strong character does not represent the paragraph direction.

Unicode UAX #9 rule HL1 explicitly permits a higher-level protocol to set the paragraph embedding level instead of applying the P2/P3 first-strong heuristic. This is also the model exposed by HTML dir="ltr", dir="rtl", and dir="auto", and by CSS direction. Supplying a base direction is not a bidi override: Latin and numeric runs remain LTR inside an RTL paragraph, while neutral resolution, visual ordering, alignment, and editing use the requested paragraph level.

Add BaseDirection to AnalysisOptions, default it to Auto, and use it in BidiResolver. Explicit RTL analysis now bypasses the all-LTR content fast path so neutral and numeric text receives complete bidi levels, and empty text retains its requested base direction. Re-export the shared parlance::BaseDirection, add Analysis::is_rtl, and expose set_base_direction on all three Parley builders. A private shared builder-options value keeps the new paragraph option and the existing build settings on one path.

This updates the unreleased low-level API: replace None, Some(0), and Some(1) passed to BidiResolver::resolve with BaseDirection::Auto, BaseDirection::Ltr, and BaseDirection::Rtl. Existing AnalysisOptions literals can add base_direction or use ..AnalysisOptions::default().

Paired Tango benchmarks against main found no consistent layout regression. The first 8-second full comparison put all 18 layout cases between -1.33% and +0.79%; a repeat ranged from -0.39% to +1.79% without a consistent pattern. A focused 20-second comparison measured the shortest default Latin case at +0.00% and the styled Latin case at -1.05%.

Spec references:

• https://www.unicode.org/reports/tr9/#HL1
• https://html.spec.whatwg.org/multipage/dom.html#the-dir-attribute
• https://www.w3.org/TR/css-writing-modes-3/#text-direction

[waywardmonkeys]

Assisted by GPT 5.6 Sol high. I reviewed it.

[waywardmonkeys]

This came about during development of Overstory and is needed there.

[nicoburns]

This makes sense to me. I believe I will eventually need a span-level override for web compat (at which point it probably makes sense to make it a style). But I imagine that's a lot more involved to implement.

[waywardmonkeys]

There's plenty of future work to do on bidi for sure. But this is fixing something really basic / simple / trivial.

[tomcur]

This looks good, thank you!

[tomcur]

I'd normally suggest dropping this method, as it's derivable from Analysis::paragraph_level (and having both methods introduces some mental overhead of whether the two methods are the same).

But, users having to is_multiple_of(2) really isn't so nice, so I'll open a follow-up proposal instead.

[waywardmonkeys]

There's more issues there than that ... paragraph level has at least 2 names in the code. :)