test(history): concept comparison for prompt history

history-concept.spec.mjs compares the Prompt history with the concept's
#prompt/alpine on the concept fixture at 1600x1100 and 390x844, both
themes and motion settings, on Chromium and touch, with its behaviors
in both documents and a mutation check. inventoryOf gains a history
section; mapHistoryConcept applies D24, D35, D45, D46, D47 and the
approved Show archived button, laying the concept out at runtime.

Corrections it demonstrated: each compared negative mounts in a
display: contents child of the concept's .compare-item (D25), and the
concept fixture now links images oldest first and counts one pair use
per image, so newest-first reads and lane run counts match the concept.
history-canonical-comparison.mjs uses the concept's selectors for
production and drops its closed polish entries.

web/js/workspace/library.mjs

@@ -486,9 +486,10 @@ export function mountLibrary(main,api,parentScope,{positiveId}={}) {
             const disposeCompareBlocks=()=>{for(const item of compareBlocks)item.dispose();compareBlocks=[];};
             content.cleanup(disposeCompareBlocks);
             function compareBlock(text,label) {
-                const item=el('div',undefined,'compare-item');
+                // The concept's `.compare-item` holds the block itself; the blocks mount in a transparent child (D25).
+                const item=el('div',undefined,'compare-item'),mount=el('div');item.append(mount);
                 // D46: the two blocks are named Baseline and Selected, so no region or action name repeats on the page.
-                compareBlocks.push(mountPromptBlocks(item,{prompts:{negative:{status:'resolved',text,source:'library'}},roles:['negative'],sender,parentScope:content,includePairAction:false,contextLabel:label,title:displayTitle(detail.positive)}));
+                compareBlocks.push(mountPromptBlocks(mount,{prompts:{negative:{status:'resolved',text,source:'library'}},roles:['negative'],sender,parentScope:content,includePairAction:false,contextLabel:label,title:displayTitle(detail.positive)}));
                 compareBlocks.at(-1).setState({prompts:{negative:{status:'resolved',text,source:'library'}},actionsEnabled:savedReadState==='current'});
                 return item;
             }

web/js/workspace/workspace.css

@@ -219,6 +219,9 @@ pre.prompt-text { font: inherit; font-size: 12px; line-height: 1.75; white-space
 /* #249 D24: the history summary's prompt blocks sit in `article.surface` as the concept's do (app.js a598); the mount
    point keeps its "Saved prompt texts" region name. */
 .library-history article.surface > .prompt-inspector { display: contents; /* D24 */ }
+/* #249 D25: each compared negative's block sits in the concept's `.compare-item` (app.js a600), so its mount point is
+   transparent and adds no margin. */
+.compare-item > .prompt-inspector { display: contents; /* D25 */ }
 /* #249 D23: a real title can be long or one unbroken word; it wraps inside the title row (the toolkit's buttons never
    wrap), while the category chip keeps whole words beside it and wraps only a name wider than its share of the row. */
 .card-title button { min-width: 0; white-space: normal; overflow-wrap: anywhere; /* D23 */ }