fix(Unit): use the exact value for the acre unit

[spokodev]

Problem

The acre unit is stored as a rounded 4046.86 m2, while the units it is defined from are stored at full precision (sqyd: 0.83612736, sqft: 0.09290304, sqmi: 2589988.110336). An acre is exactly 4840 square yards = 4046.8564224 m2, so the rounding makes conversions internally inconsistent:

math.evaluate('1 acre to sqyd') // 4840.004278773989   (expected 4840)
math.evaluate('1 acre to sqft') // 43560.0385089659    (expected 43560)
math.evaluate('1 sqmi to acre') // 639.999434212204    (expected 640)
math.evaluate('1 acre to m^2')  // 4046.86             (expected 4046.8564224)

The relative error is ~8.8e-7 — far larger than IEEE-754 noise, since 4840 * 0.83612736 is 4046.8564224 exactly.

Fix

Store the exact value, matching the precision of the units the acre is defined from:

acre: { ..., value: 4046.8564224 } // 4840 sqyd

(The international acre = 4840 international square yards, consistent with mathjs's sqyd/sqft/sqmi. The survey-based sqrd/sqch are a separate question and are left unchanged.)

Verification

• New test in Unit.test.js asserting 1 acre == 4840 sqyd == 43560 sqft; fails before, passes after.
• Unit.test.js: 130 passing, 0 regressions.
• After the fix, 1 acre to sqyd / to sqft / 1 sqmi to acre are exact (relative error from 8.8e-7 to ~1e-16).