fix(stdlib): rfind_in empty returns clamped end

crates/zz_codegen/src/runtime/strings.c

@@ -1589,7 +1589,7 @@ zz_value zz_str_rfind_in(zz_value s, zz_value sub, zz_value start, zz_value end,
     size_t needle_len = sub.s->len;
     size_t lo, hi;
     clamp_span_c((int64_t)s.s->len, start.i, end.i, &lo, &hi);
-    if (needle_len == 0) return (zz_value){ZZ_INT, {.i = (int64_t)lo}};
+    if (needle_len == 0) return (zz_value){ZZ_INT, {.i = (int64_t)hi}};
     const char *fin = src + hi;
     const char *p = src + lo;
     int64_t best = -1;

crates/zz_stdlib/src/natives/str_mod/mod.rs

@@ -261,7 +261,7 @@ fn find_in(hay: &[u8], needle: &[u8], start: i64, end: i64) -> i64 {
 fn rfind_in(hay: &[u8], needle: &[u8], start: i64, end: i64) -> i64 {
     let (s, e) = clamp_span(hay.len() as i64, start, end);
     if needle.is_empty() {
-        return s as i64;
+        return e as i64;
     }
     match memchr::memmem::rfind(&hay[s..e], needle) {
         Some(rel) => s as i64 + rel as i64,

docs/stdlib.md

@@ -93,7 +93,9 @@ import std.str
 | `bytes.to_ints` | `bytes.to_ints(b: bytes) -> [int]` | Opaque byte buffer as plain ints |
 
 Offsets are bytes (O(1) per call, O(n) streaming total; matches Rust
-`str::find` semantics). `length`/slicing stay char-oriented — convert
+`str::find` semantics). Empty `sub`: `find`/`rfind` return the clamped
+`from`, `find_in` the clamped start, `rfind_in` the clamped end,
+`count_in` returns 0; `_at` never matches empty. `length`/slicing stay char-oriented — convert
 explicitly when mixing. Negative inputs clamp to 0; empty `sub` returns
 the clamped position for `find`/`rfind` and never matches for `_at`.
 `trim_span` strips Unicode White_Space identically on VM and AOT.

tests/fixtures/stdlib/str_find_test.zz

@@ -49,6 +49,7 @@ func main() {
     println("in13={str.find_in(hay, hello_pat, 1, 12)}")
     println("inmiss={str.find_in(hay, hello_pat, 14, 24)}")
     println("inempty={str.find_in(hay, empty_pat, 3, 9)}")
+    println("rinemptyin={str.rfind_in(hay, empty_pat, 3, 9)}")
     println("rin0={str.rfind_in(hay, hello_pat, 0, 24)}")
     println("rin13={str.rfind_in(hay, hello_pat, 0, 12)}")
     println("rinmiss={str.rfind_in(hay, hello_pat, 14, 24)}")