Build HistoryStore — opt-in git and mode detection

The pro-m1 foundation card. SwiftGitX 0.4.0 (bundled libgit2, the
pathfinder's pin) joins the one target; new Kanban/Git/ holds
BoardGitMode (pure nearest-.git-wins detection, .git-as-file counts,
NSString ancestor walk), HistoryStore (@MainActor @Observable;
compose() is the tier gate — free tier gets no object, no detection,
no stat), GitRepository (scope-confined SwiftGitX handles: create =
init + HEAD forced to main + whole-tree "Initial board state" commit;
branch reads incl. unborn/detached; path-history ranks), GitIdentity
(derived default as a pure function + repo-local config reader — not
libgit2's merged ladder), and GitPathHistory (Mutex-guarded lazy
ranker). beginSession composes the git state beside the tier and
feeds BoardStore.makeIdentityHistoryRanker; git-mode loads pass the
git-backed IdentityHistoryRanker to BoardLoader. The popover's git
slot resolves a pure five-way matrix: free tier unchanged (absent /
BoardGitNote), Pro mode-aware — Add Git on mode none, honest prose on
repo-nested, read-only branch line on git. Provider binding
unchanged: both tiers still bind native until the undo/redo card.

42 new tests across 8 suites, all repositories built through bundled
libgit2; InertGitTests untouched and green. 2194 tests / 375 suites.

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

KanbanTests/BoardInfoPopoverTests.swift

@@ -31,3 +31,49 @@ struct BoardInfoPopoverTests {
         #expect(!BoardGitNote.hasGitDirectory(at: fixture.root))
     }
 }
+
+/// **The popover's git slot, posture by posture** (03-board-ui.md ▸ Board popover; 06-history-undo.md
+/// ▸ Rules; 12-editions.md ▸ The free tier and `.git`).
+///
+/// `BoardGitSection.resolve` is the whole decision, pulled out as a pure function of the tier and the
+/// board's mode precisely so the matrix is assertable — the views it selects are SwiftUI and stay
+/// untested, exactly as the note's wording and placement do above.
+@Suite("Board popover ▸ the git section's posture")
+struct BoardGitSectionTests {
+
+    @Test("The free tier: absent on an ordinary board, a Pro pointer on a board carrying an inert .git")
+    func theFreeTierIsContextual() {
+        #expect(BoardGitSection.resolve(tier: .free, mode: .none, hasGitDirectory: false) == .absent)
+        #expect(BoardGitSection.resolve(tier: .free, mode: .none, hasGitDirectory: true) == .proPointer)
+    }
+
+    @Test("Pro: mode none offers add-git, git mode shows the branch")
+    func proFollowsTheMode() {
+        #expect(BoardGitSection.resolve(tier: .pro, mode: .none, hasGitDirectory: false) == .addGit)
+        #expect(BoardGitSection.resolve(tier: .pro, mode: .git, hasGitDirectory: true) == .branch)
+    }
+
+    @Test("A repo-nested board explains itself — the add-git action is absent, not disabled")
+    func repoNestedExplainsRatherThanDisables() {
+        let section = BoardGitSection.resolve(tier: .pro, mode: .repoNested, hasGitDirectory: false)
+
+        // The design is insistent here: "not a hidden 'add git' but a short explanation … the option
+        // is absent because it *can't* apply, and the UI should teach that rather than look broken"
+        // (06 ▸ Rules). A `.addGit` that rendered disabled would satisfy neither half.
+        #expect(section == .repoNested)
+        #expect(section != .addGit)
+    }
+
+    @Test("Every posture is reachable, and none of them is two postures")
+    func theMatrixIsTotal() {
+        let resolved = Set(
+            Tier.allCases.flatMap { tier in
+                BoardGitMode.allCases.flatMap { mode in
+                    [true, false].map { BoardGitSection.resolve(tier: tier, mode: mode, hasGitDirectory: $0) }
+                }
+            }
+        )
+
+        #expect(resolved == Set(BoardGitSection.allCases))
+    }
+}