fix: round() and mod follow the XPath 1.0 numeric spec

[gaoflow]

round() and the mod operator both diverge from XPath 1.0 (and from libxml2/lxml).

round() — REC §4.4

int(math.Round(f)) rounds halves away from zero and drops non-finite values:

• round(-0.5) → -1 (want 0), round(-1.5) → -2 (want -1), round(-2.5) → -3 (want -2); §4.4 rounds halves toward +Infinity.
• round(1 div 0) → 9223372036854775807, round(-1 div 0) → the int64 minimum, round(0 div 0) → 0; want Infinity / -Infinity / NaN.
• The int result isn't an XPath number, so round(2.5) + 0.5 evaluates to NaN instead of 3.5 (asNumber only understands float64).

round() now returns float64 like ceiling()/floor(): NaN/±Infinity pass through, and finite values use floor(f) with a half-up carry. It no longer depends on the go1.10-only math.Round, so it moves next to roundFunc. (§4.4 notes that floor(f + 0.5) is wrong for the signed-zero cases, e.g. round(-0.4) is -0.)

mod — REC §3.5

float64(int(a) % int(b)) truncates the operands and panics on a zero divisor: 5.5 mod 2 → 1 (want 1.5) and 5 mod 0 panics with "integer divide by zero" (want NaN). math.Mod is the truncating IEEE remainder the spec calls for — the sign follows the dividend, and a zero divisor yields NaN.

All results checked against lxml/libxml2. The existing round tests are updated to the number result, and table tests are added for both classes covering the half-to-+Infinity, non-finite, and zero-divisor edges.

go test ./..., go test -race ./..., go vet ./..., git diff --check.

[coveralls]

coverage: 80.477% (+0.06%) from 80.418% — gaoflow:fix-round-mod-numeric-spec into antchfx:master