Fix CSS step timing functions jump-both, jump-start, start at timestamp 0; Early return when possible

[yezhizhen]

• The keyframe selection used <= for normal direction, causing the first keyframe to be selected at progress 0; changed to < for normal direction (kept <= for reverse to avoid Fix CSS step timing functions jump-both, jump-start, start at timestamp 0; Early return when possible #342 (comment)) and added direction‑aware step‑function handling.
• Guard against division‑by‑zero for zero‑length keyframe intervals.

Make early return happen earlier:

1. total_progress ≥ 1.0 and total_progress < 0 now handled immediately. Also make it direction aware.
2. At timestamp 0, for normal direction, non‑jump step functions (jump‑end, jump‑none, end) now return the start‑keyframe value early, instead of proceeding to interpolation.
3. Zero-interval: we exit early to avoid division‑by‑zero in percentage_between_keyframes

Try
Servo PR: servo/servo#44365

[yezhizhen]

@Loirooriol Just friendly reminder.

[Loirooriol]

Using debug_unreachable may be clearer.
Also the message isn't providing much, it should rather explain why.

[yezhizhen]

Done in 2701332

The explanation is a bit lengthy tho. We are relying on

stylo/style/servo/animation.rs

Lines 175 to 176 in 40aa59d

	debug_assert!(intermediate_steps.first().unwrap().start_percentage == 0.);
	debug_assert!(intermediate_steps.last().unwrap().start_percentage == 1.);

[Loirooriol]

Why can this handle the equality, but above you check progress < 0.0?

[yezhizhen]

In 2701332 I updated to ==1.0 as it is always in [0,1].

but above you check progress < 0.0?

At progress == 0.0, we don't want to early return because we need to reach the step‑function handling block at L878 we added.

[Loirooriol]

But don't we have the same jumping problem at the end?

[yezhizhen]

servo/servo#43086

End has always been fine. We only have problem at the beginning.