lib/fmt: render i64::MIN via math.absi64, not in-i64 negate (#67)

i64dec computed the magnitude with `n = -n` inside i64, which wraps at
i64::MIN (-MIN == MIN stays negative), so the `for (n > 0)` loop never
ran and println(i64::MIN) emitted only the bare '-'. The whole print/
println/fprint family was affected at the single boundary value (and
int MIN, since int is 8B). ref/hare/fmt/print.ha:124-129 takes the
magnitude through math::absi64 into a u64; do the same (math.absi64
exists, lib/math/math.ww:12). ww's own printf path was already correct
((-v): u64), so print and printf now agree.

fmttest gains fprinti64min + fprintintmin pinning the boundary.

lib/fmt/fmt.ww

@@ -22,6 +22,7 @@
 package fmt;
 
 import io;
+import math;
 import memio;
 import os;
 import strings;
@@ -38,16 +39,18 @@ let i64dec_buf: [21]u8;
 // of strconv.i64tos for the pre-printf-family print path; the
 // printf-family path below dispatches through strconv directly.
 fn i64dec(v: i64) str = {
-	let neg: bool = false;
-	let n: i64 = v;
-	if (n < 0) { neg = true; n = -n; };
+	// ref/hare/fmt/print.ha:124-129: magnitude via math::absi64 into u64,
+	// sign tested separately. `n = -n` on an i64 wraps at i64::MIN
+	// (-MIN == MIN), so the old loop never ran and only the '-' printed.
+	let neg: bool = v < 0;
+	let n: u64 = math.absi64(v);
 	let tmp: [20]u8;
 	let i: i32 = 0;
-	if (n == 0) { tmp[0] = '0'; i = 1; };
-	for (n > 0) {
-		let d: i64 = n % 10i64;
-		tmp[i] = (d + 48i64): u8;
-		n = n / 10i64;
+	if (n == 0u64) { tmp[0] = '0'; i = 1; };
+	for (n > 0u64) {
+		let d: u64 = n % 10u64;
+		tmp[i] = (d + 48u64): u8;
+		n = n / 10u64;
 		i += 1;
 	};
 	let out: i32 = 0;

lib/fmt/fmttest.ww

@@ -76,6 +76,44 @@ fn errsource() io.stream = {
 	match (c) { case void => {}; case let eioe: io.error => abort(); };
 };
 
+// ---- fprint: i64::MIN / int MIN whole-magnitude (#67) ------------------
+// i64dec used `n = -n` within i64, which wraps at i64::MIN (-MIN == MIN),
+// so the digit loop never ran and only the bare '-' was emitted. The fix
+// takes the magnitude through math.absi64 into u64. These rows pin the
+// boundary value plus a normal negative, zero, and int MIN.
+
+@test fn fprinti64min() void = {
+	let mem: memio.stream = memio.dynamic();
+	let s: io.stream = &mem.vt;
+
+	let m: i64 = -9223372036854775807i64 - 1i64;
+	let r: (size | io.error) = fmt.fprint(s, m);
+	match (r) {
+	case let n: size => { assert(!(n: i32 != 20)); };
+	case let eioe: io.error => abort();
+	};
+	assert(!(!streq(memio.string(&mem), "-9223372036854775808")));
+
+	let c: (void | io.error) = io.close(s);
+	match (c) { case void => {}; case let eioe: io.error => abort(); };
+};
+
+@test fn fprintintmin() void = {
+	let mem: memio.stream = memio.dynamic();
+	let s: io.stream = &mem.vt;
+
+	let m: int = -9223372036854775807 - 1;
+	let r: (size | io.error) = fmt.fprint(s, m, -7i64, 0i64);
+	match (r) {
+	case let n: size => { assert(!(n: i32 != 25)); };	// "-9223372036854775808 -7 0"
+	case let eioe: io.error => abort();
+	};
+	assert(!(!streq(memio.string(&mem), "-9223372036854775808 -7 0")));
+
+	let c: (void | io.error) = io.close(s);
+	match (c) { case void => {}; case let eioe: io.error => abort(); };
+};
+
 // ---- fprint: bool + rune renders as "true A" --------------------------
 
 @test fn fprintboolrune() void = {