[P2] Random number generation: rand, rand-int, seeding

[ecto]

There is no randomness primitive today. This blocks tests, sims,
property-based testing, and most game/graphics demos.

Proposal

Expose randomness as a standard effect (consistent with IO,
Time, etc.) to keep it explicit and mockable:

[Rand.int 100]
[Rand.float]
[handle expr [Rand.int n] [resume 42]]

Add an entry point for seeding so tests can be deterministic.

Acceptance criteria

• Effect declared in loon-lang with a default handler in the runtime.
• Documented in ref/effects.loon.
• A test that uses a custom handler to make Rand.int deterministic.