fix(mcp-server): review fixes for the version cards and History

The shell's version cards (shell/node-versions-source.ts):
- a version's change and undo target are the state it replaced, the
  History entry right before its first copy, not the card before it: after
  a restore those differ, and undo went to the wrong version (and not
  where restore_version steps_back=1 goes)
- a card restores the newest copy of its content, as restore_version
  does, so page settings the user changed (a "user" copy) are kept
- a blank page after a drawing is a clear of the canvas, a version of its
  own; only the blank page before any drawing is hidden
- numbers and changes are keyed by content, not by the first copy's id,
  so a version keeps them when its first copy drops out of the server's
  20-entry buffer; the caches start over for another server state (the
  process restarted: entry ids name other content)

The server's History (src/history.ts):
- firstCopyIds compares each entry with the first of every group only: a
  bare model matches any page name, so "same content" is not transitive,
  and a card could show one document and restore another
- the time and pages fields had no reader; pages parsed every XML once
  more on every write

Reading History (src/http-server.ts, shell/mcp-sync-core.ts):
- GET /api/state and a push's answer carry a History key (entry count,
  newest id, the entry on the canvas); the shell reads History again only
  when it changes, so a hand edit no longer downloads every entry's XML and
  thumbnail
- a failed History read is told again at the next poll
- a History list from a state the poll has not seen yet is dropped

packages/mcp-server/tests/http-server.test.ts

@@ -1197,8 +1197,6 @@ describe("GET /api/history for the version cards", () => {
             svg: "",
             xml: page("a"),
             source: null,
-            time: a.time,
-            pages: 1,
             firstId: a.id,
         })
         expect(d.entries[1]).toMatchObject({
@@ -1209,6 +1207,49 @@ describe("GET /api/history for the version cards", () => {
         })
     })
 
+    it("gives the state a History key, which a hand edit changes once and a further one not at all", async () => {
+        // The shell reads History (every entry's XML and picture) again
+        // only when the key changes
+        const id = "mcp-history-key"
+        setState(id, page("a"))
+        addHistory(id, page("a"))
+        const [a] = getHistory(id)
+        const first = JSON.parse(
+            (await request(`/api/state?sessionId=${id}`)).body,
+        )
+        expect(first.historyKey).toBe(`1:${a.id}:${a.id}`)
+        // A hand edit: the canvas is at no entry now, History is the same
+        const push = await postJson("/api/state", {
+            sessionId: id,
+            xml: page("b"),
+            baseVersion: first.version,
+            stateId: first.stateId,
+        })
+        expect(push.status).toBe(200)
+        expect(JSON.parse(push.body).historyKey).toBe(`1:${a.id}:null`)
+        const again = await postJson("/api/state", {
+            sessionId: id,
+            xml: page("c"),
+            baseVersion: first.version + 1,
+            stateId: first.stateId,
+        })
+        expect(JSON.parse(again.body).historyKey).toBe(`1:${a.id}:null`)
+        const polled = JSON.parse(
+            (await request(`/api/state?sessionId=${id}`)).body,
+        )
+        expect(polled.historyKey).toBe(`1:${a.id}:null`)
+        // A write keeps the edit in History: the key names the new entry
+        keepInHistory(id, page("c"))
+        setState(id, page("d"))
+        addHistory(id, page("d"))
+        const [, c, d] = getHistory(id)
+        expect(c.xml).toBe(page("c"))
+        const after = JSON.parse(
+            (await request(`/api/state?sessionId=${id}`)).body,
+        )
+        expect(after.historyKey).toBe(`3:${d.id}:${d.id}`)
+    })
+
     it("names the entry a restored copy stands for, and marks the copy as a restore", async () => {
         const id = "mcp-history-cards-restore"
         addHistory(id, page("a"))