feat: add timeline-driven video rendering with ffmpeg export

[AsPulse]

Time-driven scene composition that's rendered into a video via ffmpeg, plus the supporting type machinery for animation work.

• tellur-core::time: Time trait abstracting over TimelineTime (received from the renderer) and LocalTime (produced by remaps). Default combinators include during (gate, type-preserving), during_ripple (gate + rebase), lerp (affine warp with extrapolation), fps (floor quantization), and cycle / bounce / phase (periodic and range decompositions). A TimeOptionExt mirrors them on Option<T> for chaining.
• tellur-core::phase: Phase newtype enforcing [0.0, 1.0] at construction. Useful for easing, progress, and other normalized scalars.
• tellur-core::interpolate: Interpolate trait with impls for f32, Vec2, and Anchor. Phase::interpolate(from, to) reads naturally in animation code.
• tellur-core::timeline: Timeline trait (build(t, target) -> RasterImage) plus a timeline() closure helper.
• tellur-core::geometry: AnchoredSize::snap_to(target_point) for absolute positions (Vec2-interpolation-friendly); snap_to_anchor(target_size, target_anchor) kept as the convenience for static layouts.
• tellur-renderer::video: FfmpegEncoder that streams raw RGBA frames into an ffmpeg subprocess. Input side (size, fps) is fixed; codec / container / filters are caller-controlled raw args, so mp4/mov/ProRes/etc. are all reachable. Two-row indicatif progress bars (Render / Encode) show by default; -progress pipe:1 drives the Encode bar.
• Example: timeline_to_mp4 — four BouncingDot instances stacked vertically, each fed the same t quantized to 60/30/24/16 fps, so the lower-fps stutter is directly visible against the smooth top track.