Dice: replace std::mt19937 with a small generator

[DustinHab]

Dice keeps a std::mt19937 as a member. That is 624 words of state, and seeding it from a std::seed_seq in the constructor needs a 2504 byte stack frame, measured with -fstack-usage. The constructor runs on the display task, which is created with 800 words, so 3200 bytes for the whole call chain. It fits, but not by much.

A dice roll does not need a Mersenne twister. This swaps it for xorshift32, which holds four bytes of state.

What it buys:

	before	after
constructor stack frame	2504 B	88 B
generator state	2496 B	4 B
build size		832 B smaller

std::uniform_int_distribution still does the range mapping, so the roll logic is unchanged.

Checked on the host, since a broken generator still compiles: 600000 d6 rolls stay within 0.7 % of even, a d99 reaches both 1 and 99, two different seeds give different sequences, a zero seed does not lock the generator into its fixed point, and the sequence does not repeat within two million values. Rolling in the simulator gives varying results and colours.

Paddle and Twos already use plain rand(), so Dice was the only screen carrying a heavyweight generator.

[github-actions]

Build size and comparison to main:

Section	Size	Difference
text	387072B	-912B
data	936B	0B
bss	22936B	0B

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