Wire up Linux and Windows exports

export_presets.cfg is hand-written (no editor GUI available here): a
minimal preset per platform (name, platform, path, x86_64
architecture) is enough, since Godot fills every other option from
its platform defaults. tools/setup.sh now also fetches the matching
export templates (~1.2GB, cached outside the repo like the Godot
binary itself) -- note the directory it unpacks into uses a dot
before "stable" while .godot-version and the download URLs use a
hyphen, two different spellings of the same version that are easy to
conflate.

Both make export-linux and make export-windows now actually run and
produce a working package; tests/ and addons/gut/ are excluded from
the exported .pck (124KB vs 1.5MB before -- a dev-only dependency has
no business shipping to a child's machine). The Linux binary was
smoke-tested headless and boots cleanly; the Windows .exe could only
be confirmed as a valid PE32+ binary, since no Windows machine or
Wine is available in this environment -- flagged in
docs/testing/manual-checklist.md rather than claimed as verified.

This closes out every step in the launch prompt's "Ordre de
construction." README status lines updated to reflect the first
playable prototype rather than a bare skeleton.

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

README-EN.md

@@ -2,7 +2,7 @@
 
 An educational French-language game for 7-9 year olds (French grades CE1-CE2): an offline 2D adventure with an explorable map, a guide character, and grammar, spelling, and dictation exercises.
 
-Open source project, in development. Current status: technical skeleton (see `docs/specs/decisions.md` for scoping decisions and `docs/specs/cahier-des-charges.md` for the full specification, both in French — this is a French-curriculum product).
+Open source project, in development. Current status: first playable prototype (scope of cahier-des-charges §15) — boot, profiles, a three-location map, all three exercise types, saves, Linux/Windows exports. Pedagogical content is still minimal (see `docs/specs/decisions.md` for scoping decisions and `docs/specs/cahier-des-charges.md` for the full specification, both in French — this is a French-curriculum product).
 
 ## Tech stack
 
@@ -27,7 +27,7 @@ make run     # runs the game
 | `make validate-data` | validates the JSON files in `data/` |
 | `make test` | headless tests |
 | `make check` | lint + validate-data + test, must pass before every commit |
-| `make export-linux` / `make export-windows` | exports into `build/` (from the export step onward) |
+| `make export-linux` / `make export-windows` | exports into `build/` |
 
 ## Structure