Implement legacy -webkit-line-clamp (without ellipsis)

[nicoburns]

Summary

Implements the legacy -webkit-line-clamp recipe (display: -webkit-box; -webkit-box-orient: vertical; -webkit-line-clamp: N) in Blitz. Ellipsis rendering is intentionally out of scope — content is clamped cleanly at the Nth line boundary (equivalent to max-lines: N; block-ellipsis: none).

Depends on DioxusLabs/stylo#13, which enables -webkit-line-clamp, -moz-box-orient/-webkit-box-orient, display: -webkit-box/-webkit-inline-box, and Stylo's adjust_for_webkit_line_clamp style adjustment for Servo builds. Cargo.toml pins the stylo crates to that fork commit via git/rev; this should move back to a crates.io version once released.

How it works

Stylo's style adjuster (matching Blink) detects the full legacy recipe at computed-value time and adjusts display to flow-root (or inline-block for -webkit-inline-box), leaving original_display as the -webkit-box value. So no -webkit-box flex layout is needed — the element lays out as a normal block, and Blitz only adds the N-line height clamp in layout/inline.rs:

// applies iff original_display.inside() == WebkitBox && display.inside() != WebkitBox
// (i.e. the stylo adjustment fired: full recipe present)
if line_count > N {
    height = height.min(layout.get(N - 1).metrics().block_max_coord);
}

Lines past the Nth don't contribute to content height; with overflow: hidden they're clipped, with overflow: visible they still paint (matching Blink's legacy quirk). The gating on original_display makes -webkit-line-clamp a no-op on elements that aren't legacy webkit boxes (per WPT webkit-line-clamp-001/002).

table.rs/traversal.rs gain DisplayInside::WebkitBox match arms (treated as flow) since the enum variant now exists in Servo builds — this covers the unadjusted case where -webkit-box-orient isn't vertical.

Verification

• cargo fmt / clippy --workspace / check --workspace / test --workspace pass.
• New fixture examples/assets/line_clamp.html rendered via cargo run --example screenshot: clamp-2 + overflow: hidden shows exactly 2 lines; clamp-3 + overflow: visible has 3-line height with later lines painting past the background (Blink quirk).

• WPT css/css-overflow/line-clamp: 35 passing (up from 31 on main), including webkit-line-clamp-001 through 004, 015, 023, 029, 033, 034, 038, 039. Remaining failures are references containing an ellipsis (out of scope), script-driven tests, or line counting in block descendants (not part of this scope).

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/4a6c323b22734b559aa869fdf89fb375
Requested by: @nicoburns

WPT results

4 newly passing, 5 newly failing (net -1).

Full diff (9 changed tests)

+ Fail => Pass css/css-flexbox/balance/webkit-box-no-balance.html
- Pass => Fail css/css-overflow/line-clamp/line-clamp-018.html
+ Fail => Pass css/css-overflow/line-clamp/line-clamp-029.html
- Pass => Fail css/css-overflow/line-clamp/webkit-line-clamp-018.html
- Pass => Fail css/css-overflow/line-clamp/webkit-line-clamp-019.html
+ Fail => Pass css/css-overflow/line-clamp/webkit-line-clamp-023.html
- Pass => Fail css/css-overflow/line-clamp/webkit-line-clamp-026.html
- Pass => Fail css/css-overflow/line-clamp/webkit-line-clamp-048.html
+ Fail => Pass css/css-overflow/line-clamp/webkit-line-clamp-with-line-height.html

_{Generated by the WPT workflow.}

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