[P3] Time as a standard effect (now, epoch, formatting)

[ecto]

The hyperpolyglot ml sheet's entire dates/time section is none
for Loon today: no current time, no unix epoch, no date types.

Proposal

Add a Time standard effect (parallels IO, Rand):

[Time.now]            ; Instant
[Time.epoch]          ; Int  (seconds since 1970-01-01)
[Time.format inst "%Y-%m-%d"]  ; Str

Effects-based design keeps it testable (mock with a handler).

Acceptance criteria

• Effect, builtin, or both — declared and documented in
  ref/effects.loon.
• A test that overrides Time.now via a handler.