feat: expose buffered ranges on VideoPlayerState across all platforms

[itboy87]

Summary

Adds a cross-platform buffering API to VideoPlayerState, so consumers can draw a buffer
indicator behind the seek bar instead of guessing how much media is ready to play.

Every backend funnels its raw values through a shared normalizeBufferedRanges helper, so
callers get one predictable shape: sorted, non-overlapping, clamped to the duration, with
empty/non-finite entries dropped.

API

New in commonMain:

• BufferedRange(start, end) — an @Immutable span in seconds, with duration and contains.
• VideoPlayerState.bufferedRanges: List<BufferedRange> — the buffered spans, sorted and merged.
• VideoPlayerState.bufferedPercentage: Float — how far the media is buffered ahead of the
  current position, 0f..100f.
• VideoPlayerState.bufferedSliderPos: Float — the same value on the 0f..1000f scale used by
  sliderPos, so it can be drawn directly under the seek bar.
• VideoPlayerState.isBufferedRangeSupported: Boolean — whether the backend can report buffering
  at all. Check it first: false means "unknown", not "nothing buffered".
• List<BufferedRange>.bufferedEndAt(position) / .bufferedPercentageAt(position, duration).

All members have interface defaults (false / emptyList() / 0f), so this is source-compatible
for anyone implementing VideoPlayerState. PreviewableVideoPlayerState gained matching
constructor parameters for previews and tests.

Platform implementations

Platform	Source	Notes
Android	Player.getBufferedPosition	ExoPlayer exposes a single contiguous buffer, so at most one range, starting at the playback position
iOS	AVPlayerItem.loadedTimeRanges	KVO observer, fires as the buffer grows — including while paused
macOS (desktop)	AVPlayerItem.loadedTimeRanges	New nGetBufferedRanges JNI entry point
Linux	GStreamer buffering query	New nGetBufferedRanges JNI entry point; local files report the whole duration
Windows — HLS	IMFMediaEngine buffered ranges	Full multi-range support
Windows — local files	whole duration	Always available on disk
Windows — progressive network	—	Not supported: the Media Foundation source reader exposes no buffering data, so isBufferedRangeSupported is false
Web (JS / Wasm)	HTMLMediaElement.buffered	Driven by progress, seeked, loadedmetadata, timeupdate; reset on emptied

Implementation details worth flagging:

• Buffering is polled on its own job (200 ms) on Android, Linux, macOS and Windows, separate
  from the position-update loop — the buffer keeps filling while playback is paused, but the
  position loop is stopped in that state.
• Native calls degrade gracefully. The desktop backends catch UnsatisfiedLinkError from
  nGetBufferedRanges and permanently flip isBufferedRangeSupported to false, so a bundled
  native library that predates this change reports "unsupported" instead of throwing on every poll.
  bufferedRangesAvailable is backed by mutableStateOf so that flip recomposes.
• The JVM DefaultVideoPlayerState forwards all three members explicitly to its platform
  delegate — because they have interface defaults, omitting them would silently report "no
  buffering support".
• Native ranges cross the JNI boundary as a flat [start0, end0, start1, end1, …] DoubleArray
  capped at 16 ranges, decoded by a shared decodeNativeBufferedRanges helper.
• Web TimeRanges indexing is guarded, since the browser can mutate the object between the length
  check and the read.

Native changes

New nGetBufferedRanges JNI entry points and their implementations for Linux
(NativeVideoPlayer.c / jni_bridge.c), macOS (NativeVideoPlayer.swift / jni_bridge.c) and
Windows (NativeVideoPlayer.cpp / HLSPlayer.cpp / jni_bridge.cpp). The prebuilt native
libraries need to be rebuilt for the desktop backends to report ranges; until then they fall
back to "unsupported" rather than failing.

Sample app

The sample's seek bar now draws the buffered ranges as a track behind the slider (the inactive
track was made more translucent so they stay readable), plus a one-line readout under the position
text — e.g. Buffered 42% · 0:00–1:12 — which distinguishes "nothing buffered yet" from "this
backend cannot report buffering".

Tests

BufferedRangeTest (commonTest, 11 tests) covers duration/contains, bufferedEndAt including
gaps and empty lists, percentage clamping and unknown/NaN durations, and normalization: sorting,
merging overlapping and touching ranges, dropping empty and non-finite entries, and clamping to
the duration.

Docs

README_VIDEO.MD gains a Buffered Ranges section with a usage snippet, a property table and
the per-platform support table above, plus a feature-list bullet and a TOC entry.