feat: content-create and content-update with schema validation

All-or-nothing writes: validation (fields, terms, featured image, parent,
template) before any write; failed extras delete the new draft (create) or
restore revision, old columns, fields, terms and featured image (update).
content-update requires rev; Rev::ofPost covers content, title, excerpt,
status and post_modified_gmt. MetaFieldSource::write() now throws on failure.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01At3V6SjwbMe6eMzVGW3Hs7

tests/Unit/Abilities/RevAndJsonMapTest.php

@@ -8,6 +8,7 @@ use LIT\MCP\Abilities\JsonMap;
 use LIT\MCP\Abilities\Rev;
 use LIT\MCP\Tests\Unit\TestCase;
 use stdClass;
+use WP_Post;
 
 final class RevAndJsonMapTest extends TestCase
 {
@@ -18,6 +19,26 @@ final class RevAndJsonMapTest extends TestCase
         self::assertNotSame(Rev::of('abc'), Rev::of('abd'));
     }
 
+    public function testPostRevCoversContentTitleExcerptStatusAndModification(): void
+    {
+        $base = [
+            'post_content' => '<p>A</p>',
+            'post_title' => 'T',
+            'post_excerpt' => 'E',
+            'post_status' => 'draft',
+            'post_modified_gmt' => '2026-09-25 08:00:00',
+        ];
+        $rev = Rev::ofPost(new WP_Post($base));
+
+        self::assertMatchesRegularExpression('/^[0-9a-f]{12}$/', $rev);
+        self::assertSame($rev, Rev::ofPost(new WP_Post($base)));
+        self::assertNotSame(Rev::of('<p>A</p>'), $rev);
+        foreach (['post_content' => '<p>B</p>', 'post_title' => 'U', 'post_excerpt' => 'F',
+            'post_status' => 'publish', 'post_modified_gmt' => '2026-09-25 08:00:01'] as $key => $value) {
+            self::assertNotSame($rev, Rev::ofPost(new WP_Post(array_merge($base, [$key => $value]))), $key);
+        }
+    }
+
     public function testEmptyMapBecomesJsonObject(): void
     {
         self::assertInstanceOf(stdClass::class, JsonMap::of([]));