Initial project import

tools/v/vlib/math/decimal/README.md

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+## Description
+
+`math.decimal` provides arbitrary-precision fixed-point decimal arithmetic.
+It stores values as an integer coefficient plus a decimal scale, so addition,
+subtraction, and multiplication stay exact.
+
+Division is exposed through `div_prec`, which rounds half up to a caller-chosen
+number of decimal places. The `/` operator uses
+`decimal.default_division_precision`.
+
+## Example
+
+```v
+import math.decimal
+
+price := decimal.from_string('19.99')!
+fee := decimal.from_string('1.50')!
+total := price + fee
+assert total.str() == '21.49'
+
+share := total.div_prec(decimal.from_int(3), 2)!
+assert share.str() == '7.16'
+```