Implement undo and redo as forward commits

GitHistoryProvider is the second HistoryProviding implementation:
its stack IS HEAD's first-parent ancestry, reseeded on load (redo
empty), re-synced to HEAD before every crossing so agents'
self-commits become the top and ⌘Z steps back exactly one commit;
any arriving commit clears redo (a heal-only window deliberately
does not). Restores are forward commits through the ordinary
signature path — GitRestoreOperation materializes only the
current-vs-target diff as working-tree writes and resolves no
reset/checkout symbol at all; heal commits are transparent
in-session (pointer passes over, restores exclude heal-owned paths,
identity carried on landed windows via PlannedCommit.kind →
GitLandedCommit). Subjects "Undo:/Redo: <crossed subject>"; menu
labels never nest in-session; the root commit is not a step
(crossing it would restore the empty tree).

Provider binding flips: makeHistoryProvider(store, tier, git) —
free binds native everywhere, Pro binds the git provider on git
boards and NOTHING on mode-none/repo-nested (the pair disables
through existing validation); add-git mid-session live-binds via
HistoryStore.didAddGit → bindHistoryProvider (the flip only ever
adds).

SessionSettleGate is the reusable Save All / Discard / Cancel step:
restores whose diff touches an open Edit session or raw-source
buffer gate on it (Save All applies with validation — a refused
buffer cancels the whole restore focused on the offender; Discard
reverts via CardBodyEditSession.discardBuffer and reconciles against
the working tree, deliberately skipping the second flush); untouched
sessions ride through undisturbed. Built for the branch-switch card
to reuse. BoardStore gains the async performWholesale sibling.

CardHistorySection fills the m6 EmptyView slot: read-only, newest
first, follows the card across lane moves by folder-component match
(the UUID is the identity — no rename detection), absent off git
mode and off Pro.

2332 tests / 403 suites green; InertGitTests untouched.

Claude-Session: https://claude.ai/code/session_01SR4XGjmBE16ZUYWpfFHXwY

Kanban/UI/Card/CardWindowView.swift

@@ -91,6 +91,10 @@ struct CardWindowView: View {
     let comments: CardComments
     /// This window's thumbnail memory, held by the host so it outlives a snapshot.
     let thumbnails: AttachmentThumbnailCache
+    /// **This card's commit trail** (05 ▸ History), or `nil` on every board with no app-managed git —
+    /// the free tier, mode none, and repo-nested boards. The `nil` *is* the section's absence rule;
+    /// see `historySlot`.
+    let history: CardHistory?
     /// The whole-window file drop (05 ▸ Attachments: "the drop surface remains the **whole
     /// window**"). `nil` only where a caller has no store to import through.
     let fileDrop: CardWindowDropDelegate?
@@ -325,22 +329,21 @@ struct CardWindowView: View {
         }
     }
 
-    /// **The History section's reserved place in the stack** — between Details and Actions, 05's
-    /// order (05 ▸ History: "the card's commit trail, read-only … newest first — semantic subject,
-    /// relative date, author").
+    /// **The History section** — between Details and Actions, 05's order (05 ▸ History: "the card's
+    /// commit trail, read-only … newest first — semantic subject, relative date, author").
     ///
-    /// Nothing is drawn yet, deliberately: the section is conditional on a git mode that does not
-    /// exist here, so a header over empty space would claim a commit trail on every board — and on
-    /// the boards where it is *absent* by design (mode none, repo-nested) it would be claiming one
-    /// that can never arrive. What the slot reserves is the **position**, so filling it in moves
-    /// nothing above or below it.
+    /// **Absence is the `nil`, and it is the whole rule.** "The section is absent on boards without
+    /// app-managed git (mode none, repo-nested) — same honesty rule as the popover's git section",
+    /// and the free tier has no git state at all (12-editions.md ▸ The free tier and `.git`). The host
+    /// builds a `CardHistory` only in mode `git`, so there is no placeholder here to decide about:
+    /// what the slot reserves is the **position**, and on every other board that position is empty.
     ///
-    // m7-git: the trail itself, plus the two rules that come with it — absence on boards without
-    // app-managed git (the same honesty rule as the board popover's git section, 06-history-undo.md)
-    // and View ▸ History, which focuses this section (11-command-nexus.md).
+    // A later card: View ▸ History, which focuses this section (11-command-nexus.md).
     @ViewBuilder
     private var historySlot: some View {
-        EmptyView()
+        if let history {
+            CardHistorySection(history: history)
+        }
     }
 }