feat(newgame): one public, seeded roll the screen can show

roll_attributes(seed) is now the single implementation of the die — construct
calls it, and the creation screen (M4-b) will call it to DISPLAY the roll. So
the screen shows real numbers while still only ever handing over a seed (§2).
validate() goes public so the screen can gate its CTA on the real rules.

Neither takes a default parameter (traps.md #2).

client/tests/unit/test_new_game.gd

@@ -300,3 +300,50 @@ func test_non_companion_override_goes_to_state_not_log():
 	for m in res["log"].to_dict()["party"]:
 		assert_ne(m["id"], "oda_fenn")
 	assert_eq(res["log"].party_member("brannoc_thane").disposition, 40)
+
+
+func test_roll_attributes_is_pure_and_seeded():
+	# Same seed, same five. The screen shows these; construct builds from them.
+	var a := NewGame.roll_attributes(8675309)
+	var b := NewGame.roll_attributes(8675309)
+	assert_eq(a, b)
+	assert_eq(a.keys(), Attributes.IDS, "the five rows, in the contract's order")
+	assert_false(a.has("lck"), "LCK is not an attribute and never leaves construct (§7)")
+
+
+func test_roll_attributes_holds_the_floor_of_eight():
+	# Sweep enough seeds that a straight 3d6 would certainly have rolled below 8
+	# somewhere. (GUT has no assert_gte — assert_true on the comparison.)
+	var lowest := 999
+	for s in range(400):
+		for stat in Attributes.IDS:
+			lowest = mini(lowest, int(NewGame.roll_attributes(s)[stat]))
+	assert_true(lowest >= 8, "the floor broke: something rolled %d" % lowest)
+	assert_eq(lowest, 8, "the floor should actually BITE across 2000 rolls, not sit unused")
+
+
+func test_construct_builds_exactly_what_roll_attributes_showed():
+	# TRAP 1 (traps.md): this is the load-bearing claim of the creation SCREEN —
+	# "the numbers you saw are the numbers you got". Asserting that two fresh
+	# rolls agree would pass even if construct kept its own second implementation
+	# of the die. This asserts across the PIPELINE: what roll_attributes returns,
+	# plus the spend, IS what construct puts on the sheet.
+	var spend := {"str": 2, "con": 1}
+	var res := _build(_creation({"seed": 4242, "spend": spend}))
+	assert_true(res["ok"], str(res["errors"]))
+
+	var shown := NewGame.roll_attributes(4242)
+	var expected := {}
+	for stat in Attributes.IDS:
+		expected[stat] = int(shown[stat]) + int(spend.get(stat, 0))
+
+	assert_eq(res["state"].sheet.attributes, expected)
+
+
+func test_validate_is_public_and_agrees_with_construct():
+	var bad := _creation({"calling_id": "paladin"})
+	var errors := NewGame.validate(_deserter(), world, bad)
+	assert_gt(errors.size(), 0)
+	var res := _build(bad)
+	assert_false(res["ok"])
+	assert_eq(res["errors"], errors, "the screen must be told exactly what construct would reject")