T0: optimize thumbnail requests and reject obsolete preview work

Skip unchanged thumbnail jobs and reuse bounded settings hashes (both maps
capped at 1024 entries). Reject stale render results and cache writes after
edits, library changes and preview clears: render requests carry a request
id and cache generation, PreviewCache writes and reads hold the generation
lock that clear() takes, and library.clearPreviews also drops cached
sources. The web worker index follows the same generation rule.

The decoder guards, Windows path fixes and test fixture changes that were
bundled in a327b98 are separate commits; the benchmark follows this one.

Co-authored-by: Codex <noreply@openai.com>
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/library.rs

@@ -991,6 +991,7 @@ pub fn specs() -> Vec<CommandSpec> {
         }),
         cmd!("library.clearPreviews", "Clear Preview Cache", ["File"], None, "{}", always, |s, _| {
             s.media.rendered.clear();
+            s.media.clear_sources();
             ok()
         }),
     ]