fix(render): clear asset caches between watch renders

[LeadcodeDev]

Severity Low, category coherence. Location: crates/rustmotion/src/cli/commands/render.rs:452

Impact

Triggering scenario: rustmotion render deck.json -o out.mp4 --watch left open for an afternoon, with an image component pointing at logo.png. The author re-exports logo.png. ASSET_CACHE is keyed on the path alone (image.rs:41-54) with no mtime or size component, and in the incremental branch clear_asset_cache() only runs when the video config hash changes (resolution/fps) — editing scene content never clears it. Every subsequent re-render silently re-uses the stale decoded bitmap; the author sees the old logo indefinitely with nothing on stderr. The adjacent audio path solved exactly this and documents why (encode/audio_analysis.rs:29-39: fingerprint on len+mtime because 'someone re-exports a mix while the studio is open') — the image/GIF/video caches never got the same treatment. Memory side: clear_asset_cache touches only ASSET_CACHE; GIF_CACHE, VIDEO_FRAME_CACHE and AUDIO_ANALYSIS_CACHE have no clear function at all, so a long --watch session that touches several asset variants accumulates every decode for the process lifetime.

Fix

Key ASSET_CACHE/GIF_CACHE/VIDEO_FRAME_CACHE on (path, len, mtime) the way source_fingerprint already does for audio, or add a clear_all_caches() invoked at the top of every watch iteration. The first is strictly better: it keeps incremental reuse for untouched assets.

Evidence the audit read

let use_prev = if prev_config_hash == Some(config_hash) {
    prev_segments.as_deref()
} else {
    engine::clear_asset_cache();
    None
};
// assets.rs:24
pub fn clear_asset_cache() {
    if let Some(cache) = ASSET_CACHE.get() { cache.clear(); }
}

Stacked on fix/gif-decode-bomb, which carries the previous finding of this workstream. GitHub shows only this finding's diff; merge in order.

Part of the September 2026 audit remediation chantier. Refs #220 (RM-25).