Fix line-box content detection for negative-height boxes, NBSP width handling and inline box quantization

[nicoburns]

LLM Contributions: This PR was produced with Devin (an LLM tool) under the direction of the requester; the follow-up regression triage of Blitz #832 is documented on that PR.

Three fixes for regressions found while triaging the WPT diff of DioxusLabs/blitz#832 (which consumes #18).

1. LineBoxMetrics::add_inline_box (parley/src/layout/line_break.rs) treated an inline box with ascent + descent <= 0 as "no content", so a line containing only an atomic inline with a large negative margin-top (reserved height negative but finite) was laid out as an empty/invisible line with no strut. Now any finite non-zero extents count as content; only out-of-flow boxes (which Blitz passes with NEG_INFINITY extents) are still skipped.

2. add_inline_box also rounded ascent and descent independently when quantize is set. For a box whose baseline splits it into two .5 halves this reserves one more pixel than the box's (already integral) height, e.g. 82px for an 81px box. Inline box extents are exact sizes from the caller, not font metrics, so they are no longer rounded (the box height and the resulting line coords are still quantized as before). Together with (1) this fixes css/CSS2/floats-clear/floats-141.xht.

3. calculate_content_widths (parley/src/layout/data.rs) subtracted NBSP advances as "trailing whitespace" when computing min/max content widths. NBSP is neither collapsible nor hangable (css-text-3 §4.1.1, §4.1.3), so a shrink-to-fit inline-block containing only   collapsed to zero width. Only U+0020 is trimmed now. Fixes inline-block-zorder-004/005.xht and t41-html4-keywords-a.xht in Blitz WPT. The line-breaker's own is_space check at line-break time is intentionally left as is_space_or_nbsp(): changing it made 8 justify/trailing-space tests pass but regressed line-break-anywhere-overrides-uax-behavior-004/006, so that is left for a separate change.

Regression tests added in parley/src/tests/test_style_metrics.rs:

• line_with_negative_height_inline_box_is_not_invisible
• quantized_inline_box_extents_are_not_rounded_separately
• no_break_spaces_are_not_hanging_whitespace

Changelog: None

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/5c4401ef6292427a952400c36ae85272
Open in Devin Desktop: https://dioxus.staging.devinenterprise.com/desktop/session/5c4401ef6292427a952400c36ae85272?variant=devin-insiders
Requested by: @nicoburns

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[staging-devin-ai-integration]

Folded into #18 (commits 9ac58e1, 7991b17, 5f301bf cherry-picked onto its branch).