feat(cursor): add a step path easing so a caret jumps instead of gliding

[LeadcodeDev]

What

cursor draws a caret — a rounded vertical bar. cursor_style is metadata today: "default" and "pointer" render identically, as the source already notes.

Every path_easing it offered interpolated (linear, ease_out, ease_in_out), so a caret moving between two text fields slid across the gap and spent most of the transition outside any field. A text caret never does that — it holds a position and jumps.

Why it cannot be worked around in a scenario

auto_path_offset interpolates with a Catmull-Rom whose tangent reads waypoints[seg_idx + 2]. On a segment whose two endpoints are the same point — the obvious way to express "hold this position" — that neighbour still bends the curve:

p_prev = p1 = p2 = 336, p3 = 736
catmull_rom(t) = 336 - 200t² + 200t³     → -29.6 px at t = 2/3

So the caret swings ~30 px backwards before returning, mid-hold. Expressing a hold-then-jump without this easing takes four identical waypoints per stop — twelve for three fields — plus reasoning about click_duration short-circuiting the intermediate segments. That is a lot of scenario to say "don't interpolate".

step sets t = 0.0, and catmull_rom(0, …) returns p1 exactly whatever its neighbours are, so it is immune to this by construction.

How

CursorPathEasing::Step holds the departure waypoint for the whole segment; the move happens when time reaches the next waypoint and the segment changes. One match arm, no change to the existing easings, which stay the default — this is additive.

Tests

• step holds the first waypoint across the segment and lands on the next
• linear still interpolates (the addition does not change existing behaviour)
• "step" parses from JSON in snake_case

Docs

SKILL.md lists the new value, and now says plainly that the component draws a caret and that cursor_style does not change the drawing — the table previously described it as "cursor appearance style", which reads as though it did.

cargo test --workspace green, cargo fmt --check and cargo clippy --all-targets clean.