Add pure correction and pedagogy logic with full test coverage

scripts/correction/: AnswerValidator (multiple choice, sentence
correction via tap-the-faulty-word + retype) and DictationAnalyzer
build the CorrectionResult structure from cahier des charges §6.
Dictation analysis aligns words with WordAligner (Wagner-Fischer edit
distance) before any character comparison, so an omitted or added
word doesn't cascade into false errors on every following word.
TextNormalizer handles non-breaking spaces, typographic apostrophes,
and decomposed-vs-composed French accented characters via an
explicit table (GDScript has no built-in NFC/NFD normalization).
Error classification follows the data annotations only (rule_id
present -> grammatical, absent -> unclassified), never a runtime
linguistic guess.

scripts/pedagogy/: DifficultyManager (per-skill difficulty from
recent results), AttemptFlow (3-attempt graduated aid: hint, then
explanation, then reveal, matching exercise-lifecycle.md's second
gap), and ReviewScheduler (the five mastery states from §9, with
session-counted spaced review intervals [1, 3, 6, 12] rather than
wall-clock dates, since a family device's clock isn't trustworthy).

All of it is pure: no scene tree, no clock or RNG read directly,
every effectful input (session number, attempt outcome) is a
parameter. 65 headless tests across both layers, including the
word-omission-doesn't-cascade regression and the full
decouverte -> acquise -> a_reviser -> acquise path.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/pedagogy/test_difficulty_manager.gd

@@ -0,0 +1,32 @@
+extends GutTest
+## Tests de DifficultyManager.
+
+
+func test_three_consecutive_successes_raise_difficulty() -> void:
+    var recent: Array[bool] = [true, true, true]
+    assert_eq(DifficultyManager.next_difficulty(1, recent), 2)
+
+
+func test_three_consecutive_failures_lower_difficulty() -> void:
+    var recent: Array[bool] = [false, false, false]
+    assert_eq(DifficultyManager.next_difficulty(2, recent), 1)
+
+
+func test_mixed_results_keep_difficulty_stable() -> void:
+    var recent: Array[bool] = [true, false, true]
+    assert_eq(DifficultyManager.next_difficulty(2, recent), 2)
+
+
+func test_difficulty_never_exceeds_maximum() -> void:
+    var recent: Array[bool] = [true, true, true]
+    assert_eq(DifficultyManager.next_difficulty(3, recent), 3)
+
+
+func test_difficulty_never_goes_below_minimum() -> void:
+    var recent: Array[bool] = [false, false, false]
+    assert_eq(DifficultyManager.next_difficulty(1, recent), 1)
+
+
+func test_not_enough_history_keeps_difficulty_unchanged() -> void:
+    var recent: Array[bool] = [true, true]
+    assert_eq(DifficultyManager.next_difficulty(2, recent), 2)