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>

scripts/correction/correction_result.gd

@@ -0,0 +1,35 @@
+class_name CorrectionResult
+extends RefCounted
+## Structure de correction retournee par answer_validator et dictation_analyzer
+## (cahier des charges §6, docs/specs/decisions.md point 4). Chaque entree de
+## `errors` : {"position": int, "error_type": String ("lexical" | "grammatical" |
+## "unclassified" | "capitalization"), "rule_id": String, "found": String,
+## "expected": String}.
+
+var is_correct: bool
+var errors: Array[Dictionary]
+var explanation: String
+var hint: String
+var remediation: Array[String]
+
+
+static func correct(explanation: String) -> CorrectionResult:
+    var result := CorrectionResult.new()
+    result.is_correct = true
+    result.errors = []
+    result.explanation = explanation
+    result.hint = ""
+    result.remediation = []
+    return result
+
+
+static func incorrect(
+    errors: Array[Dictionary], explanation: String, hint: String, remediation: Array[String]
+) -> CorrectionResult:
+    var result := CorrectionResult.new()
+    result.is_correct = false
+    result.errors = errors
+    result.explanation = explanation
+    result.hint = hint
+    result.remediation = remediation
+    return result