fix: address review findings on the slide-execution hooks

[turegjorup]

Review follow-ups for #5, plus the scheduling doc and the template skill update. Targets feature/template-cleanups, not release/3.0.0.

Hooks

• Empty entries now finish the slide — moved into useMultipleEntrySlideExecution, declared per template via emptyEntriesDuration (existing timings kept: 1s, 5s for news-feed, 2s for slideshow). Each template had its own copy of the timer, which is the duplication the extraction removes elsewhere; a new template that omits it locks the playlist.
• Cycling starts when entries arrive after run — the effect keyed only on run, so a feed resolving late was skipped for that rotation.
• The hook returns the clamped entryDuration — poster, instagram-feed and slideshow derived their fade timers from the raw prop, so duration: 0 left the hook cycling at 15s while the fade fired immediately and the entry sat faded out. clampDuration extracted to slide-utils/duration.js so both clocks agree.
• Dropped durationRef from useBaseSlideExecution — read once, synchronously, in an effect recreated on every run change, so it prevented no staleness the comment claimed.

Templates

• image-text: images/duration mirrored into refs in a layout effect instead of during render.
• video: slide/slideDone read through refs, since finish() runs from a guard timer or media event; element paused on teardown so a preview toggling run stops making noise.

Tests

116 passing, 22 files. New: clampDuration, empty-feed fallback across all five consumers, late-arriving entries, the exposed clamped duration, fade-timer alignment, image-text cycling, video pause and slideDone routing.

The fade-timer and video cases were confirmed red against this branch before the fix. The image-text cases pass either way — they pin behaviour, since that change is behaviour-preserving.

vitest.config.js gains the svgr plugin from vite.config.js; without it an import X from "./x.svg" resolves to a data-URI string and any template importing an SVG throws in jsdom, so instagram-feed could not be tested at all.

Docs

• docs/client-scheduling.md — content selection, rotation, and the slideDone contract. Also documents two things easy to miss: a staged list goes live immediately when nothing is playing (wraparound is the swap point only mid-rotation), and slides marked invalid are dropped by the region.
• add-slide-template skill — the duration field must be named duration (the Admin's widget writes to a hardcoded id, so any other name diverges from what it displays), and the Admin stores ms while showing seconds.

Not addressed

• useBaseSlideExecution.js / useMultipleEntrySlideExecution.js are camelCase while the repo uses kebab-case, including these hooks' own test files. Left alone — renaming churns every import.
• checkForEmptyContent reads a stale list (region.slide vs region.slides); noted in the doc, fix belongs elsewhere.