feat(mod_format): Add check-only formatting with interactive diff preview; make formatting default to dry-run and extend formatting API

ts/mod_format/classes.baseformatter.ts

@@ -1,6 +1,6 @@
 import * as plugins from './mod.plugins.js';
 import { FormatContext } from './classes.formatcontext.js';
-import type { IPlannedChange } from './interfaces.format.js';
+import type { IPlannedChange, ICheckResult } from './interfaces.format.js';
 import { Project } from '../classes.project.js';
 
 export abstract class BaseFormatter {
@@ -79,4 +79,94 @@ export abstract class BaseFormatter {
   protected async shouldProcessFile(filepath: string): Promise<boolean> {
     return true;
   }
+
+  /**
+   * Check for diffs without applying changes
+   * Returns information about what would change
+   */
+  async check(): Promise<ICheckResult> {
+    const changes = await this.analyze();
+    const diffs: ICheckResult['diffs'] = [];
+
+    for (const change of changes) {
+      // Skip generic changes that don't have actual content
+      if (change.path === '<various files>') {
+        continue;
+      }
+
+      if (change.type === 'modify' || change.type === 'create') {
+        // Read current content if file exists
+        let currentContent: string | undefined;
+        try {
+          currentContent = await plugins.smartfs.file(change.path).encoding('utf8').read() as string;
+        } catch {
+          // File doesn't exist yet
+          currentContent = undefined;
+        }
+
+        const newContent = change.content;
+
+        // Check if there's an actual diff
+        if (currentContent !== newContent && newContent !== undefined) {
+          diffs.push({
+            path: change.path,
+            type: change.type,
+            before: currentContent,
+            after: newContent,
+          });
+        }
+      } else if (change.type === 'delete') {
+        // Check if file exists before marking for deletion
+        try {
+          const currentContent = await plugins.smartfs.file(change.path).encoding('utf8').read() as string;
+          diffs.push({
+            path: change.path,
+            type: 'delete',
+            before: currentContent,
+            after: undefined,
+          });
+        } catch {
+          // File doesn't exist, nothing to delete
+        }
+      }
+    }
+
+    return {
+      hasDiff: diffs.length > 0,
+      diffs,
+    };
+  }
+
+  /**
+   * Display a single diff using smartdiff
+   */
+  displayDiff(diff: ICheckResult['diffs'][0]): void {
+    console.log(`\n--- ${diff.path}`);
+    if (diff.before && diff.after) {
+      console.log(plugins.smartdiff.formatLineDiffForConsole(diff.before, diff.after));
+    } else if (diff.after && !diff.before) {
+      console.log('  (new file)');
+      // Show first few lines of new content
+      const lines = diff.after.split('\n').slice(0, 10);
+      lines.forEach(line => console.log(`  + ${line}`));
+      if (diff.after.split('\n').length > 10) {
+        console.log('  ... (truncated)');
+      }
+    } else if (diff.before && !diff.after) {
+      console.log('  (file will be deleted)');
+    }
+  }
+
+  /**
+   * Display all diffs from a check result
+   */
+  displayAllDiffs(result: ICheckResult): void {
+    if (!result.hasDiff) {
+      console.log('  No changes detected');
+      return;
+    }
+    for (const diff of result.diffs) {
+      this.displayDiff(diff);
+    }
+  }
 }