Complete the Sentence quiz type: engine, UI, tests

Add a new Complete the Sentence quiz type that renders a Spanish example
sentence from the card with the target word blanked out and asks the
student to pick the missing word from 4 choices (other cards' fronts
from the same week's pool).

Core logic lives in SharedModels/SentenceQuizEngine as pure functions
over VocabCard, covered by 18 Swift Testing tests. CourseQuizView calls
the engine, pre-filters the card pool to cards that can produce a
resolvable blank, and reuses the existing MC rendering via a new
correctAnswer(for:) helper.

VocabCard gains examplesBlanks (parallel array to examplesES) so content
can explicitly tag the blanked substring; DataLoader reads an optional
"blank" key on each example. Additive schema change, CloudKit-safe
default.

Also adds ContentCoverageTests that parse the repo's course_data.json
and assert every card has >=3 examples and yields a resolvable question.
These tests currently fail: 1,117 cards still need sentences. They are
the oracle for the gap-fill pass that follows.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Conjuga/SharedModels/Package.swift

@@ -3,11 +3,15 @@ import PackageDescription
 
 let package = Package(
     name: "SharedModels",
-    platforms: [.iOS(.v18)],
+    platforms: [.iOS(.v18), .macOS(.v14)],
     products: [
         .library(name: "SharedModels", targets: ["SharedModels"]),
     ],
     targets: [
         .target(name: "SharedModels"),
+        .testTarget(
+            name: "SharedModelsTests",
+            dependencies: ["SharedModels"]
+        ),
     ]
 )