Books — fix infinite render loop via value-based navigation

Opening a book chapter froze the app in an infinite render loop. Root
cause: the books screens used the eager `NavigationLink { destination }`
form inside `List`/`LazyVStack`. That form keeps the destination view
structurally parented to the source row, so `BookReaderView`'s ScrollView
got trapped inside a `List` row — a row sizes to intrinsic height, a
ScrollView has none, so the two never converge and re-measure forever.

Switch the whole books navigation chain to value-based navigation:
- practiceHomeView, BookLibraryView, BookChapterListView use
  NavigationLink(value:).
- PracticeView's NavigationStack declares the BooksRoute, Book, and
  BookChapter destinations once, at the stack root (mixing eager and
  value-based pushes in one path caused pushed screens to pop back).
- BookReaderView is built from just a BookChapter; it resolves its Book
  by slug via @Query.

Also:
- BookChapter gains a stored paragraphCount so the chapter list no longer
  decodes the full paragraph JSON on every render (bookDataVersion -> 6
  to re-seed).
- BookSpeechController builds its AVSpeechSynthesizer lazily.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Conjuga/SharedModels/Sources/SharedModels/BookChapter.swift

@@ -10,6 +10,9 @@ public final class BookChapter {
     public var bookSlug: String = ""
     public var number: Int = 0
     public var title: String = ""
+    /// Spanish paragraph count, stored at seed time so chapter lists can show
+    /// it without decoding the full `paragraphsESJSON` blob on every render.
+    public var paragraphCount: Int = 0
     public var paragraphsESJSON: Data = Data()
     public var paragraphsENJSON: Data = Data()
 
@@ -18,6 +21,7 @@ public final class BookChapter {
         bookSlug: String,
         number: Int,
         title: String,
+        paragraphCount: Int,
         paragraphsESJSON: Data,
         paragraphsENJSON: Data
     ) {
@@ -25,6 +29,7 @@ public final class BookChapter {
         self.bookSlug = bookSlug
         self.number = number
         self.title = title
+        self.paragraphCount = paragraphCount
         self.paragraphsESJSON = paragraphsESJSON
         self.paragraphsENJSON = paragraphsENJSON
     }