Navigate the poem tree instead of the visit history

Breadcrumbs: a poem's path (poet » book » section) is now built from its own
fullTitle and fullUrl and every ancestor is tappable, so the trail is the same
whether the poem was opened by browsing, from the saved list, from a
breadcrumb, or by reading on from the previous poem.

Back: browsing is a tree, so Back climbs it — poem to section to book to poet
to home — rather than retracing however you arrived. Each screen derives its
own parent from its URL and navigation keeps the stack flat, which is what
stops Back from walking forward again into the page you just left. System Back
and the top-bar arrow do the same thing.

The exception is a poem opened from the saved list: that was reached from a
list, not a shelf, so Back returns to the list, and reading on through the
divan keeps that origin.

Plus a Home button on every screen below the poet list.

Verified on an API 36 emulator: drilled Hafez > ghazals > ghazal 3 and walked
back up to the poet list, and opened a saved Khayyam rubai and confirmed Back
returns to the saved list rather than the poet's section.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app/src/test/java/com/ganjoor/android/CoupletsTest.kt

@@ -1,6 +1,9 @@
 package com.ganjoor.android
 
+import com.ganjoor.android.data.Crumb
 import com.ganjoor.android.data.Verse
+import com.ganjoor.android.data.breadcrumbs
+import com.ganjoor.android.data.parentUrl
 import com.ganjoor.android.data.couplets
 import org.junit.Assert.assertEquals
 import org.junit.Test
@@ -66,3 +69,59 @@ class CoupletsTest {
         assertEquals(listOf(2, 1, 1), grouped.map { it.size })
     }
 }
+
+class BreadcrumbsTest {
+    @Test
+    fun `every ancestor is linked and the poem itself is not`() {
+        val crumbs = breadcrumbs("حافظ » غزلیات » غزل شمارهٔ ۱", "/hafez/ghazal/sh1")
+
+        assertEquals(listOf("حافظ", "غزلیات", "غزل شمارهٔ ۱"), crumbs.map { it.label })
+        assertEquals(listOf("/hafez", "/hafez/ghazal", null), crumbs.map { it.url })
+    }
+
+    @Test
+    fun `deeply nested books keep the whole trail`() {
+        val crumbs = breadcrumbs(
+            "مولانا » مثنوی معنوی » دفتر اول » بخش ۱",
+            "/moulavi/masnavi/daftar1/sh1",
+        )
+
+        assertEquals(4, crumbs.size)
+        assertEquals("/moulavi/masnavi/daftar1", crumbs[2].url)
+        assertEquals(null, crumbs.last().url)
+    }
+
+    @Test
+    fun `a title and url that disagree produce nothing, so the caller falls back`() {
+        assertEquals(emptyList<Crumb>(), breadcrumbs("حافظ » غزل ۱", "/hafez/ghazal/sh1"))
+        assertEquals(emptyList<Crumb>(), breadcrumbs("", "/hafez/ghazal/sh1"))
+    }
+
+    @Test
+    fun `a poem directly under a poet still links the poet`() {
+        val crumbs = breadcrumbs("حافظ » ساقی‌نامه", "/hafez/saghinameh")
+
+        assertEquals(listOf("/hafez", null), crumbs.map { it.url })
+    }
+}
+
+class ParentUrlTest {
+    @Test
+    fun `each level climbs to the one above it`() {
+        assertEquals("/hafez/ghazal", parentUrl("/hafez/ghazal/sh1"))
+        assertEquals("/hafez", parentUrl("/hafez/ghazal"))
+        assertEquals("/moulavi/masnavi", parentUrl("/moulavi/masnavi/daftar1"))
+    }
+
+    @Test
+    fun `a poet root has no parent, so the caller sends you home`() {
+        assertEquals(null, parentUrl("/hafez"))
+        assertEquals(null, parentUrl("hafez"))
+        assertEquals(null, parentUrl(""))
+    }
+
+    @Test
+    fun `trailing slashes don't invent a level`() {
+        assertEquals("/hafez", parentUrl("/hafez/ghazal/"))
+    }
+}