feat: add absorb command — merge source entity into target

DB: Absorb() merges body (newline-separated), unions tags, demotes
crystallized sources, soft-deletes source. Rejects crystallized targets.

API: POST /api/entities/:id/absorb { source_id }

CLI: nib absorb <target> <source> with prefix ID resolution

Web: absorb button on fluid entities, 'a' keyboard shortcut,
source picker modal

web/style.css

@@ -483,6 +483,33 @@ main {
     font-family: var(--font-mono);
 }
 
+/* Absorb modal */
+.absorb-list {
+    max-height: 300px;
+    overflow-y: auto;
+}
+
+.absorb-source-item {
+    display: flex;
+    align-items: center;
+    gap: 10px;
+    padding: 8px 12px;
+    cursor: pointer;
+    border-radius: var(--radius);
+    transition: background 0.1s;
+}
+
+.absorb-source-item:hover {
+    background: var(--bg-hover);
+}
+
+.absorb-source-item .entity-body {
+    font-size: 13px;
+    white-space: nowrap;
+    overflow: hidden;
+    text-overflow: ellipsis;
+}
+
 /* Responsive */
 @media (max-width: 900px) {
     main { grid-template-columns: 1fr; }