feat(sync): resolve §11 idempotency via canonical comparison + corpus harness

Close Задача №0 (SPEC §11) with the spec-sanctioned option (b): compare a
canonicalized ProseMirror form instead of raw bytes.

- canonicalize.ts: canonicalizeContent/docsCanonicallyEqual — strip node attrs.id,
  drop null/undefined attrs, and drop attrs equal to their type's known non-null
  schema default (KNOWN_DEFAULTS: link target/rel, comment.resolved, orderedList.start,
  diagram/media align) so "absent" ≡ "default"; comment anchors + meaningful attrs kept
- roundtrip.ts: assert markdown byte-stability AND canonical stability; add --corpus
  mode and mutually-exclusive-flag warning
- synthetic corpus (headings, marks, lists, table, callout, code w/ trailing \n,
  diagrams, textStyle/mention) + canonicalize/corpus tests (558 green)
- known converter asymmetries (block image after paragraph; embed width/height
  coercion) converge to a fixpoint after one export->import pass -> handled by
  normalize-on-write at vault-write time; isolated under it.fails
- SPEC §11: record the resolution and normalize-on-write strategy

test/fixtures/corpus/02-inline-marks.json

@@ -0,0 +1,62 @@
+{
+  "type": "doc",
+  "content": [
+    {
+      "type": "paragraph",
+      "content": [
+        { "type": "text", "marks": [{ "type": "bold" }], "text": "bold" },
+        { "type": "text", "text": " " },
+        { "type": "text", "marks": [{ "type": "italic" }], "text": "italic" },
+        { "type": "text", "text": " " },
+        { "type": "text", "marks": [{ "type": "code" }], "text": "code" },
+        { "type": "text", "text": " " },
+        { "type": "text", "marks": [{ "type": "strike" }], "text": "strike" }
+      ]
+    },
+    {
+      "type": "paragraph",
+      "content": [
+        {
+          "type": "text",
+          "marks": [
+            {
+              "type": "link",
+              "attrs": {
+                "href": "https://example.com/page"
+              }
+            }
+          ],
+          "text": "a link"
+        },
+        { "type": "text", "text": ", " },
+        {
+          "type": "text",
+          "marks": [{ "type": "highlight" }],
+          "text": "highlighted"
+        },
+        { "type": "text", "text": ", base" },
+        { "type": "text", "marks": [{ "type": "subscript" }], "text": "sub" },
+        { "type": "text", "text": " and base" },
+        { "type": "text", "marks": [{ "type": "superscript" }], "text": "sup" },
+        { "type": "text", "text": "." }
+      ]
+    },
+    {
+      "type": "paragraph",
+      "content": [
+        { "type": "text", "text": "Here is a " },
+        {
+          "type": "text",
+          "marks": [
+            {
+              "type": "comment",
+              "attrs": { "commentId": "cmt-xyz789" }
+            }
+          ],
+          "text": "commented anchor span"
+        },
+        { "type": "text", "text": " that must survive (SPEC §3)." }
+      ]
+    }
+  ]
+}