feat(core): add thumbnail component and text track store feature

[mihar-22]

Ref: #501
Ref: #496

Summary

Thumbnail preview core logic and text track store feature. Platform components (HTML, React) follow in stacked PRs.

• ThumbnailCore — runtime-agnostic class: binary search for active thumbnail by time, CSS constraint-based scaling (uniform scale factor from min/max-width/height), resize computation for sprite sheets via overflow: hidden + transform: translate().
• createThumbnail factory (core/dom) — manages ResizeObserver, img load/error tracking, natural dimension caching, and CSS constraint reading. Lazy binding via connect() for frameworks with deferred element availability (React refs).
• textTrackFeature + selectTextTrack — store feature that monitors TextTrackList for thumbnail/chapter tracks, producing cue arrays and the thumbnail track's base URL.
• mapCuesToThumbnails — parses VTT cues with #xywh= media fragments into ThumbnailImage[].
• findTrackElement (utils/dom) — finds the <track> element backing a TextTrack.

Sizing contract

Consumer controls size via CSS max-width/max-height on the container. ThumbnailCore.resize() reads constraints from getComputedStyle, computes a uniform scale factor, and returns container + image dimensions + sprite offsets. No constraints = native tile dimensions.

Testing

54 core tests + 7 text track feature tests, all passing.

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[github-actions]

📦 Bundle Size Report

Package	Size	Diff		%
@videojs/core	8.21 kB	+1.35 kB	████████	+19.6%	🔴
@videojs/element	1.60 kB	0 B	░░░░░░░░	0%	✅
@videojs/html	10.63 kB	+939 B	████░░░░	+9.4%	🔺
@videojs/icons	3.79 kB	0 B	░░░░░░░░	0%	✅
@videojs/react	9.69 kB	+928 B	████░░░░	+10.3%	🔴
@videojs/store	1.94 kB	0 B	░░░░░░░░	0%	✅
@videojs/utils	2.54 kB	+35 B	█░░░░░░░	+1.4%	🔺

Total: 38.41 kB · +3.20 kB · +9.1%

---

Entry Breakdown

Subpath sizes are the additional bytes on top of the root entry point, measured by bundling root + subpath together and subtracting the root-only size.

@videojs/core

Entry	Base	PR	Diff	%
.	3.21 kB	3.86 kB	+659 B	+20.0%	🔴
./dom	3.65 kB	4.36 kB	+720 B	+19.3%	🔴
total	6.87 kB	8.21 kB	+1.35 kB	+19.6%

@videojs/element

Entry	Base	PR	Diff	%
.	817 B	817 B	0 B	0%	✅
./context	823 B	823 B	0 B	0%	✅
total	1.60 kB	1.60 kB	0 B	0%

@videojs/icons

Entry	Base	PR	Diff	%
./react	2.27 kB	2.27 kB	0 B	0%	✅
./html	1.52 kB	1.52 kB	0 B	0%	✅
total	3.79 kB	3.79 kB	0 B	0%

@videojs/store

Entry	Base	PR	Diff	%
.	1.29 kB	1.29 kB	0 B	0%	✅
./html	468 B	468 B	0 B	0%	✅
./react	199 B	199 B	0 B	0%	✅
total	1.94 kB	1.94 kB	0 B	0%

@videojs/utils

Entry	Base	PR	Diff	%
./array	104 B	104 B	0 B	0%	✅
./dom	725 B	760 B	+35 B	+4.8%	🔺
./events	227 B	227 B	0 B	0%	✅
./function	197 B	197 B	0 B	0%	✅
./object	119 B	119 B	0 B	0%	✅
./predicate	265 B	265 B	0 B	0%	✅
./string	110 B	110 B	0 B	0%	✅
./style	185 B	185 B	0 B	0%	✅
./time	478 B	478 B	0 B	0%	✅
./number	158 B	158 B	0 B	0%	✅
total	2.51 kB	2.54 kB	+35 B	+1.4%

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ℹ️ How to interpret

Sizes are minified + brotli, measured with esbuild.
Package totals are computed as root size + marginal subpath costs.
Subpath marginal cost = (root + subpath bundled together) − root alone.

Icon	Meaning
✅	No change
🔺	Increased ≤ 10%
🔴	Increased > 10%
🔽	Decreased
🆕	New (no baseline)

Run pnpm size locally to check current sizes.