feat(editor): preservation schema, block reuse, toolbar, uploads and embeds

- preservationExtensions: raw HTML cards for unmodelled blocks (shown as
  text), figure with figcaption, the YouTube embed placeholder node, span
  marks, a global attribute map (never on* handlers), tables without
  colgroup or min-width, inline images without base64
- serializeDocument keeps the original bytes of every top-level block still
  equal to the loaded one, so editing one paragraph changes only that
  paragraph; tables serialize with thead and bare cell text
- Links accept only http, https, mailto and relative URLs (isPolicyUrl, the
  rule both sanitizer policies apply) and gain no target, rel or class
- Toolbar with headings, marks, lists, links, blockquote, table editing,
  images, embeds, raw HTML block, undo and redo; en and pl labels
- Images from the picker or a drop go out as summer-upload and are inserted
  on summer-upload-result with alt text and an optional caption
- editor-corpus.json: the editor own output, rebuilt and checked by
  corpus.test.ts (UPDATE_CORPUS=1 rewrites it) for the policy tests
- README Blog Editor section

assets/src/blogeditor/blogeditor.css

@@ -28,12 +28,90 @@ golem15-journal-blogeditor .blogeditor__bar {
   border-radius: 12px 12px 0 0;
 }
 
+golem15-journal-blogeditor .blogeditor__tools {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 4px 12px;
+}
+
+golem15-journal-blogeditor .blogeditor__group {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 2px;
+}
+
 golem15-journal-blogeditor .blogeditor__tabs {
   display: flex;
   gap: 2px;
   margin-left: auto;
 }
 
+golem15-journal-blogeditor .blogeditor__form {
+  display: flex;
+  flex-wrap: wrap;
+  align-items: flex-end;
+  gap: 6px 8px;
+  padding: 8px;
+  border-bottom: 1px solid var(--c-border);
+  background: var(--c-surface);
+}
+
+golem15-journal-blogeditor .blogeditor__form label {
+  display: flex;
+  flex: 1 1 220px;
+  flex-direction: column;
+  gap: 2px;
+  font-size: 12px;
+  color: var(--c-muted);
+}
+
+golem15-journal-blogeditor .blogeditor__form input {
+  box-sizing: border-box;
+  width: 100%;
+  padding: 5px 8px;
+  border: 1px solid var(--c-border-strong);
+  border-radius: 6px;
+  background: var(--c-bg);
+  color: var(--c-text);
+  font: inherit;
+  font-size: 13px;
+}
+
+golem15-journal-blogeditor .blogeditor__form input::placeholder {
+  color: var(--c-placeholder);
+}
+
+golem15-journal-blogeditor .blogeditor__form input:focus {
+  outline: 2px solid var(--c-ring);
+  outline-offset: -1px;
+}
+
+golem15-journal-blogeditor .blogeditor__form button[data-action="apply"],
+golem15-journal-blogeditor .blogeditor__form button[data-action="insert"] {
+  background: var(--c-primary);
+  color: var(--c-on-primary);
+}
+
+golem15-journal-blogeditor .blogeditor__error {
+  flex-basis: 100%;
+  margin: 0;
+  font-size: 12px;
+  color: var(--c-danger);
+}
+
+golem15-journal-blogeditor .blogeditor__error:empty,
+golem15-journal-blogeditor .blogeditor__status:empty {
+  display: none;
+}
+
+golem15-journal-blogeditor .blogeditor__status {
+  margin: 0;
+  padding: 6px 12px;
+  border-top: 1px solid var(--c-border);
+  font-size: 12px;
+  color: var(--c-muted);
+}
+
 golem15-journal-blogeditor button {
   font: inherit;
   font-size: 13px;
@@ -227,3 +305,80 @@ golem15-journal-blogeditor .ProseMirror img {
   max-width: 100%;
   height: auto;
 }
+
+golem15-journal-blogeditor .ProseMirror table {
+  border-collapse: collapse;
+  width: 100%;
+}
+
+golem15-journal-blogeditor .ProseMirror :where(td, th) {
+  border: 1px solid var(--c-border-strong);
+  padding: 4px 8px;
+  vertical-align: top;
+}
+
+golem15-journal-blogeditor .ProseMirror th {
+  background: var(--c-subtle);
+  font-weight: 600;
+  text-align: left;
+}
+
+golem15-journal-blogeditor .ProseMirror :where(td, th) > p {
+  margin: 0;
+}
+
+golem15-journal-blogeditor .ProseMirror .selectedCell {
+  background: var(--c-sel);
+}
+
+golem15-journal-blogeditor .ProseMirror figure {
+  margin-left: 0;
+  margin-right: 0;
+}
+
+golem15-journal-blogeditor .ProseMirror figcaption {
+  margin-top: 4px;
+  font-size: 13px;
+  color: var(--c-muted);
+}
+
+/* Raw HTML and embed cards: never live markup. */
+golem15-journal-blogeditor .blogeditor__raw,
+golem15-journal-blogeditor .blogeditor__embed {
+  border: 1px dashed var(--c-border-strong);
+  border-radius: 8px;
+  background: var(--c-subtle);
+  color: var(--c-muted);
+  font-size: 13px;
+}
+
+golem15-journal-blogeditor .blogeditor__embed {
+  padding: 16px;
+  text-align: center;
+}
+
+golem15-journal-blogeditor .blogeditor__card-head {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 8px;
+  padding: 4px 4px 4px 10px;
+  border-bottom: 1px dashed var(--c-border);
+}
+
+golem15-journal-blogeditor .blogeditor__raw-source {
+  max-height: 160px;
+  margin: 0;
+  padding: 8px 10px;
+  overflow: auto;
+  background: transparent;
+  font-family: "DM Mono", ui-monospace, monospace;
+  font-size: 12px;
+  white-space: pre-wrap;
+}
+
+golem15-journal-blogeditor .ProseMirror-selectednode.blogeditor__raw,
+golem15-journal-blogeditor .ProseMirror-selectednode.blogeditor__embed,
+golem15-journal-blogeditor .ProseMirror img.ProseMirror-selectednode {
+  outline: 2px solid var(--c-ring);
+}