Fixes the :half_even/:ceiling/:floor rounding half of #236.

Problem

Decimal.div/2 computes the quotient to precision + 1 digits and then discards the long-division remainder, so the rounding step cannot distinguish an exact tie from a value with more nonzero digits below the guard digit. This rounds roughly 5% of inexact divisions the wrong way:

• :half_even / :half_down — a guard digit of 5 with a nonzero tail is not a tie and must round up. 4.7460 / -5522 at precision 28 truly ends …1405287…; decimal returned …140 instead of …141.
• :ceiling / :floor / :up — a guard digit of 0 with a nonzero tail is still nonzero. :ceiling could return a result below the true quotient (and :floor above it), violating the modes' defining guarantee. Example: Decimal.div(Decimal.new("1.0000001"), Decimal.new("1")) at precision 5 :ceiling returned 1.0000 (below the true value) instead of 1.0001.

Fix

div_calc/5 already returns the remainder; this passes rem != 0 to context/3 as the sticky bit, which every rounding mode's increment?/5 already consumes (the same mechanism add/2 uses since the CVE‑2026‑32686 mitigation). One line.

The :inexact flag

The same remainder also feeds the :inexact condition. context_test.exs asserted [:rounded] (not :inexact) for 10^106 / 17 at precision 111, because :inexact was suppressed whenever the remainder happened to be a power of ten. That division is non-terminating, so the rounded result is inexact — Python's decimal flags both Rounded and Inexact. The test is updated to assert both.

Verification

Differential fuzz against Python's decimal (which passes the official GDA decarith suite): all 7 rounding modes × 6,000 random divisions each (42,000 total), 0 mismatches after the fix — every result adjudicated against the exact quotient with rational arithmetic. Before the fix, ~5% per mode diverged.

Tests

• decimal_test.exs — new div/2 regression test covering the guard‑5 sticky case, genuine ties still rounding to even, and the :ceiling/:floor directional cases.
• context_test.exs — the 10^106 / 17 flag assertion now expects :rounded and :inexact.

All existing tests pass (101 doctests, 27 properties, 102 tests).