Fix phantom positive implicit track for auto-start/definite-end placements

[nicoburns]

Objective

Fix the last two failing subtests of the WPT tests css/css-grid/parsing/grid-template-{columns,rows}-computed-implicit-track.html in Servo: with grid-template-columns: none and a child at grid-column: auto / 1, the resolved value serialized as 10px 0px instead of 10px — a phantom zero-sized positive implicit track leaked into DetailedGridTracksInfo.

Context

The bug is in the implicit grid size estimate, not serialization. In child_min_line_max_line_span, the (Auto, Line(track)) case computed min = track, ignoring that an auto-start/definite-end placement resolves to a span of 1 ending at that line (occupying [track - 1, track]):

// End track specified
- (Auto, Line(track)) => track,
+ (Auto, Line(track)) => track - 1,

(The symmetric (Line(track), Auto) max case already correctly used track + 1.)

With min under-counted, the estimate reported 0 negative implicit tracks, and the "adjust positive track estimate to fit the max span" step then compensated by adding a spurious positive implicit track. The negative track was later added correctly during placement via expand_to_fit_range, leaving one real track plus a phantom 0px track at the end (negative_implicit=1, explicit=0, positive_implicit=1). The wrong estimate also caused the auto-placement cursor to spuriously advance in the secondary axis (via the primary_span.start < primary_idx check), creating an extra implicit row as well.

Both WPT subtests are ported as gentest fixtures (grid_template_serialization_negative_implicit_{column,row}.html) — the generated expectations from Chrome assert resolved-columns="10px"/resolved-rows="10px", matching the WPT expectations exactly.

Verified: cargo test --workspace --all-features passes, cargo fmt / cargo clippy clean (one pre-existing MSRV warning unrelated to this change).

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/4946c92d12ba4edc933f5a8888f5584f
Open in Devin Desktop: https://dioxus.staging.devinenterprise.com/desktop/session/4946c92d12ba4edc933f5a8888f5584f?variant=devin-insiders
Requested by: @nicoburns

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