lib/strconv: parseint sign+overflow core; stoi64/stou64 fidelity (strconv-int fold-1 C1)

Port ref/hare/strconv/stou.ha:8-65 (rune_to_integer + parseint) and the
stoi64/stou64 fidelity rewrite (stoi.ha:9-17, stou.ha:70-76) over the old
digval loop. parseint is the shared sign + per-digit + multiply-overflow
core returning ((bool, u64) | invalid | overflow); stoi64/stou64 destructure
its `(sign, u)` tuple-in-union result — the shape unblocked by #242/#241.

Wins over the prior ad-hoc parse: leading '+' accepted, '-' on stou64 is
overflow (not silently dropped), wraparound overflow detection (n < old),
and the invalid payload carries the offending byte index per Hare.

Tests: lib/strconv/test/inttest.ww (run via test/wcc/922_strconv_int_run.c),
inline per-case checks mirroring Hare's assert sequences stoi.ha:56-86 /
stou.ha:116-138 (Hare's strconv int tests are flat sequences, not row
tables; feedback_test_match_hare_source). Covers valid dec/hex/oct/bin,
+/- sign, invalid+index, overflow, and U64_MAX / I64_MAX / I64_MIN
boundaries. The I64_MIN expectation is spelled -I64_MAX-1 (Hare's own
two's-complement identity) to isolate the test from #245 (wwstage mis-lexes
the literal 9223372036854775808 -> 0); the parse INPUT is unaffected and
yields the correct value on both stages.

combined.ww regen: strconv is compiler-imported (via fmt), so w6c +
wwdump main.combined.ww are regenerated.

test/wcc/700_e2e.c

@@ -1258,8 +1258,11 @@ static const struct row rows[] = {
 	  "  };\n"
 	  "  return acc;\n"
 	  "};", 35 },	/* 42 + (-7) + 0 (invalid at index 0 in \"abc\") */
-	/* strconv.stou64: success path; leading-sign rejected with
-	 * invalid carrying the offending index. */
+	/* strconv.stou64: success path; a leading '-' is rejected with
+	 * overflow per Hare (ref/hare/strconv/stou.ha:72-74 — parseint
+	 * accepts the sign, stou64 then rejects sign==true as overflow).
+	 * Updated from the prior ad-hoc parse, which mis-reported it as
+	 * invalid(index 0) (strconv-int fold-1 fidelity fix). */
 	{ "import strconv;\n"
 	  "type r_t = (u64 | strconv.invalid | strconv.overflow);\n"
 	  "fn main() i32 = {\n"
@@ -1272,12 +1275,12 @@ static const struct row rows[] = {
 	  "    case let e: strconv.overflow => acc += -200;\n"
 	  "  };\n"
 	  "  match (r2) {\n"
-	  "    case let v: u64 => acc += -100;\n"
-	  "    case let e: strconv.invalid => acc += e: i32;\n"
-	  "    case let e: strconv.overflow => acc += -200;\n"
+	  "    case let v: u64 => acc += 100;\n"
+	  "    case let e: strconv.invalid => acc += 200;\n"
+	  "    case let e: strconv.overflow => acc += 7;\n"
 	  "  };\n"
 	  "  return acc;\n"
-	  "};", 123 },	/* 123 + 0 (invalid at index 0 in \"-1\") */
+	  "};", 130 },	/* 123 + 7: \"-1\" is overflow (not invalid/success) */
 	/* strings.byteindex with (str | rune) needle: returns (i32 | void). */
 	{ "import strings;\n"
 	  "fn pick(r: (i32 | void), miss: i32) i32 = {\n"