Make the screen explain itself: labels, tools, help pages, hints

For developers new to submodules (notes/usability-plan.md, D32):
- x and X read "restore to pin" and "restore committed pins"
- problems only, filter and fetch are shown in the footer
- ? shows every key in titled columns, then a legend of the status
  words, symbols and spine roles; esc closes it
- u reads "pull (fast-forward)" on the superproject
- the detail pane suggests f when the last fetch is old
- a hint above the footer until the first key
- the header says "staged", like the tree

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

internal/tui/theme/legend.go

@@ -0,0 +1,62 @@
+package theme
+
+import "submarine/internal/submodule"
+
+// LegendRow is one word or symbol of the UI, rendered as it appears, and
+// what it means.
+type LegendRow struct{ Word, Meaning string }
+
+// LegendGroup is a titled column of the legend.
+type LegendGroup struct {
+	Title string
+	Rows  []LegendRow
+}
+
+// issueMeaning explains an issue word in a few plain words.
+func issueMeaning(k submodule.IssueKind) string {
+	switch k {
+	case submodule.IssueUnpushed:
+		return "pin is on no remote"
+	case submodule.IssueConflict:
+		return "merge conflict"
+	case submodule.IssueMissing:
+		return "not checked out"
+	case submodule.IssueOffPin:
+		return "checkout is not the pin"
+	case submodule.IssueChanged:
+		return "uncommitted files"
+	default:
+		return "pin not on target branch"
+	}
+}
+
+// Legend explains the words and symbols of the tree and the detail pane, in
+// their colors: what the full help's second page shows.
+func (t Theme) Legend() []LegendGroup {
+	g := t.Glyphs
+	muted := Fg(t.Muted)
+	status := LegendGroup{Title: "Status"}
+	for _, k := range submodule.Kinds {
+		status.Rows = append(status.Rows, LegendRow{t.LevelStyle(k.Level(), Fg(t.Text)).Render(IssueWord(k)), issueMeaning(k)})
+	}
+	status.Rows = append(status.Rows,
+		LegendRow{Fg(t.Info).Render(Staged), "new pin, not committed"},
+		LegendRow{muted.Render(InSync), "nothing to do"},
+	)
+	symbols := LegendGroup{Title: "Symbols", Rows: []LegendRow{
+		{Fg(t.Info).Render(g.Staged), "pin staged in parent"},
+		{muted.Render(g.Ahead + "n " + g.Behind + "n"), "ahead / behind the ref"},
+		{muted.Render("+n"), "n more issues"},
+		{Fg(t.Warn).Render(g.Problems + "k"), "issues below, collapsed"},
+		{muted.Render("[n]"), "children, collapsed"},
+	}}
+	commits := LegendGroup{Title: "Commits", Rows: []LegendRow{
+		{muted.Render(RoleWord(submodule.RolePin, "")), "what the parent records"},
+		{Fg(t.Text).Bold(true).Render(RoleWord(submodule.RoleYou, "")), "the checkout"},
+		{muted.Render(RoleWord(submodule.RoleTip, "")), "newest on the ref"},
+		{muted.Render("main"), "tip of target branch"},
+		{muted.Render(RoleWord(submodule.RoleBase, "")), "where lines split"},
+		{muted.Render(RoleWord(submodule.RoleHead, "")), "superproject checkout"},
+	}}
+	return []LegendGroup{status, symbols, commits}
+}