Add the playable scene loop: boot through mission to reward

Wires every autoload up to a real UI: boot -> profile select/create
-> world map (three mission markers, positioned from data, not
hardcoded pixels) -> mission intro (narrative from data/missions/,
a new data type with its own schema) -> rule panel -> exercise host
-> one of the three exercise type scenes -> feedback panel -> reward
screen -> back to the map.

exercise_host is the only piece with real orchestration logic: picks
exercises via ExerciseEngine (now fully implemented, validating at
load like make validate-data does), validates answers through the
pure AnswerValidator/DictationAnalyzer, escalates aid over 3 attempts
via AttemptFlow, feeds DifficultyManager so adaptive difficulty is
actually wired in rather than dead code, and records every attempt
through ProgressManager. Every exercise-type scene (multiple_choice,
sentence_correction, dictation) only emits its answer by signal, per
the architecture invariant.

All child-facing text now comes from data/localization/fr.json via
UIText, not hardcoded in scripts or scenes. Getting that right
required extending SchemaValidator to support "additionalProperties"
as a schema (dynamic keys), which also closed a real gap: save.skills
was previously unvalidated against SkillState's actual shape.

16 new headless tests, including two integration tests that instance
exercise_host.tscn directly and drive it by emitting child-scene
signals, stopping short of the final scene transition (which would
tear down the test runner's own tree). Everything that needs a
screen -- layout at different sizes, touch, virtual keyboard, actual
audio playback (no .ogg files exist yet) -- is listed in the new
docs/testing/manual-checklist.md instead of claimed as verified.

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

data/schemas/mission.schema.json

@@ -0,0 +1,33 @@
+{
+  "type": "object",
+  "required": [
+    "mission_id",
+    "title",
+    "narrative_context",
+    "rule_id",
+    "exercise_type",
+    "map_position",
+    "review_status"
+  ],
+  "additionalProperties": false,
+  "properties": {
+    "mission_id": { "type": "string", "pattern": "^[a-z][a-z0-9_]*$" },
+    "title": { "type": "string", "minLength": 1 },
+    "narrative_context": { "type": "string", "minLength": 1 },
+    "rule_id": { "type": "string", "pattern": "^[a-z][a-z0-9_]*$" },
+    "exercise_type": {
+      "type": "string",
+      "enum": ["multiple_choice", "sentence_correction", "dictation"]
+    },
+    "map_position": {
+      "type": "object",
+      "required": ["x", "y"],
+      "additionalProperties": false,
+      "properties": {
+        "x": { "type": "number", "minimum": 0, "maximum": 1 },
+        "y": { "type": "number", "minimum": 0, "maximum": 1 }
+      }
+    },
+    "review_status": { "type": "string", "enum": ["draft", "validated"] }
+  }
+}