[P1] Core math builtins missing: sqrt, pow, floor, ceil, round, trig, log, exp, pi, e

[ecto]

Loon currently has abs, min, max, sum, and integer truncation
via [int x], but no other standard math.

Proposal

Add to the builtin set (or a Math module):

• sqrt, pow (or **)
• floor, ceil, round
• sin, cos, tan, asin, acos, atan, atan2
• log, log10, exp
• Constants pi and e (or Math.pi / Math.e)

Acceptance criteria

• Available in the VM and documented in ref/builtins.loon.
• Tests covering basic values and edge cases (sqrt 0, pow 2 10,
  floor -1.5).

Found while completing the hyperpolyglot ml column.