Respect rounding mode when quantizing sub-ULP values

[chiliec]

Fixes #140

What

Context.Quantize ignored the rounding mode when the input had fewer significant digits than the target exponent (i.e. its magnitude was smaller than one unit in the last place). It truncated straight to zero regardless of the configured Rounder:

ctx := apd.Context{Precision: 100, Rounding: apd.RoundCeiling, MaxExponent: 1000, MinExponent: -1000, Traps: apd.DefaultTraps}
var d apd.Decimal
d.SetString("0.002")
ctx.Quantize(&d, &d, -1)
// got "0.0", want "0.1"

RoundCeiling(1.002, -1) already returned 1.1; only the sub-ULP case was wrong.

Fix

In the p < 0 branch of quantize, run the discarded coefficient through the context's Rounding.ShouldAddOne (the same mechanism the normal rounding path uses) instead of unconditionally zeroing it.

Tests

Added TestQuantizeRounding covering the reported case plus every rounding mode (ceiling/floor/up/down/half-up/half-even, positive and negative). Verified genuine RED→GREEN:

# with fix
$ go test -run TestQuantizeRounding .
ok  github.com/cockroachdb/apd/v3

# revert context.go, keep the test
$ git stash push context.go && go test -run TestQuantizeRounding .
--- FAIL: TestQuantizeRounding/0.002:_-1_ceiling
    expected: 0.1, got: 0.0
--- FAIL: TestQuantizeRounding/-0.002:_-1_floor
    expected: -0.1, got: -0.0
FAIL

Full suite (including the GDA conformance tests) is green:

$ go test ./...
ok  github.com/cockroachdb/apd/v3  1.524s

Happy to adjust.