Preview uncommitted changes in the detail pane

For a dirty submodule, list the changed files so the user can remember
what they were doing there:

  Changes   5 files · 2M 1A 1D 1R · +37 −12
            ──────────────────────────────────
            M  render.go  src/render/  +20 −4  3d ago
            ?  scratch.txt                     5m ago

- Status.Changes: files changed against HEAD (staged and unstaged
  combined) with kind, line counts and modification time; sorted by kind,
  then by size; capped at 200 with totals over all files.
- Git provider: parse `status --porcelain=v2 -z` and one
  `diff HEAD --numstat -z -M`, only for dirty submodules.
- Detail pane: flat list with a colored status letter per file, like IDE
  source-control views; counts and age in compact columns; up to 10 files,
  every kind keeps a line, the rest is summarized as "n more".
- Fixture gains libs/work with every kind of change; mock data extended.
- Docs: F24, change preview in the UX guide, D17.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/submodule/model.go

@@ -2,7 +2,12 @@
 // (recursively nested) git submodules. It has no UI dependencies.
 package submodule
 
-import "time"
+import (
+	"cmp"
+	"slices"
+	"strings"
+	"time"
+)
 
 // Repo is the superproject.
 type Repo struct {
@@ -100,6 +105,68 @@ type Commit struct {
 	When    time.Time
 }
 
+// ChangeKind classifies one changed file. The order is the display order.
+type ChangeKind int
+
+const (
+	ChangeConflicted ChangeKind = iota
+	ChangeModified
+	ChangeAdded
+	ChangeRemoved
+	ChangeRenamed
+	ChangeUntracked
+	NumChangeKinds
+)
+
+// MaxChangeFiles limits Changes.Files; Changes.Total keeps the real count.
+const MaxChangeFiles = 200
+
+// FileChange is one changed file, relative to HEAD of the submodule
+// (staged and unstaged changes are combined).
+type FileChange struct {
+	Path      string
+	OrigPath  string // old path, only for renames
+	Kind      ChangeKind
+	Added     int       // added lines; 0 for untracked, binary and nested submodules
+	Removed   int       // removed lines
+	Binary    bool      // line counts are unknown
+	Submodule bool      // a nested submodule pointer changed
+	Modified  time.Time // modification time of the file; zero if it doesn't exist
+}
+
+// Changes describes what differs in the worktree of a submodule.
+type Changes struct {
+	Files   []FileChange // at most MaxChangeFiles, sorted by kind, then by changed lines
+	Total   int          // all changed files; may exceed len(Files)
+	Added   int          // added lines in all files
+	Removed int          // removed lines in all files
+}
+
+// Counts returns the number of files per kind among files (all of
+// c.Files if files is nil).
+func (c *Changes) Counts(files []FileChange) [NumChangeKinds]int {
+	if files == nil && c != nil {
+		files = c.Files
+	}
+	var n [NumChangeKinds]int
+	for _, f := range files {
+		n[f.Kind]++
+	}
+	return n
+}
+
+// Sort orders Files by kind, then by changed lines (largest first), then by
+// path.
+func (c *Changes) Sort() {
+	slices.SortStableFunc(c.Files, func(a, b FileChange) int {
+		return cmp.Or(
+			cmp.Compare(a.Kind, b.Kind),
+			cmp.Compare(b.Added+b.Removed, a.Added+a.Removed),
+			strings.Compare(a.Path, b.Path),
+		)
+	})
+}
+
 // Status is the full state of one submodule.
 type Status struct {
 	Presence   Presence
@@ -111,4 +178,5 @@ type Status struct {
 	CheckedOut *Commit
 	Tip        *Commit
 	Between    []Commit // commits that make up the pin <-> tip difference
+	Changes    *Changes // nil if the worktree is clean or the details are unknown
 }