feat(selfhost): AST producer — async-let + nested-fresh return type → 459/582

- `async let name = value` (managed, no annotation, async=true) — dispatched in
  emit_stmt before the plain let; previously mis-parsed as an assignment.
- returns-fresh now also fires when `fresh` appears NESTED in the return-type region
  (e.g. `-> Result<fresh String, JsonError>`), not only immediately after `->`,
  matching parse_function_decl's region scan (the nested `fresh` still renders on
  the inner arg TypeRef; the immediate `-> fresh T` form is still consumed).

Corpus reach: 405 -> 459 / 582 byte-exact (+54, ~79%), still 0 run-failures.
Floor -> 459. New samples async_let.rss, nested_fresh.rss; 27 samples green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: olwang

crates/rsscript/src/selfhost_parity.rs

@@ -1522,7 +1522,7 @@ const AST_SAMPLE_MIN: usize = 6;
 /// Floor for `ast_parity_corpus` — the number of corpus files whose rss AST dump
 /// already matches the oracle byte-for-byte. Ratchets up as the producer's
 /// coverage grows; a drop signals a regression. (Full parity = files.len().)
-const AST_CORPUS_PARITY_FLOOR: usize = 405;
+const AST_CORPUS_PARITY_FLOOR: usize = 459;
 
 /// Step-2 measurement gate (ignored by default): how many corpus files the rss
 /// producer reproduces byte-for-byte. Not full parity yet — this ratchets a floor

selfhost/astdump.rss

@@ -1170,6 +1170,18 @@ fn emit_stmt(toks: read List<Tok>, start: read Int, limit: read Int, d: read Int
         emit(depth: read d, s: read "continue")
         return stmt_end(toks: read toks, start: read start, limit: read limit)
     }
+    // async let name = value  (managed, no annotation, async=true)
+    if is_kw_text(toks: read toks, i: read start, word: read "async") && is_kw_text(toks: read toks, i: read (start + 1), word: read "let") {
+        let e = stmt_end(toks: read toks, start: read start, limit: read limit)
+        let name = tk_text(toks: read toks, i: read (start + 2))
+        emit(depth: read d, s: read String.concat(left: read String.concat(left: read "let kind=managed name=", right: read name), right: read " mut=false async=true"))
+        let asg = first_eq(toks: read toks, start: read (start + 3), end: read e)
+        if asg >= 0 {
+            emit(depth: read (d + 1), s: read "value")
+            emit_expr(toks: read toks, start: read (asg + 1), end: read e, d: read (d + 2))
+        }
+        return e
+    }
     // let / local [mut] name [: type] = value
     if is_kw_text(toks: read toks, i: read start, word: read "let") || is_kw_text(toks: read toks, i: read start, word: read "local") {
         return emit_let(toks: read toks, start: read start, limit: read limit, d: read d)
@@ -1476,6 +1488,11 @@ fn emit_function(toks: read List<Tok>, start: read Int, d: read Int) -> Int {
     let sigEnd = function_signature_end(toks: read toks, start: read (pclose + 1))
     let effPos = find_effects_kw(toks: read toks, start: read (pclose + 1), end: read sigEnd)
     let defaultImpl = has_default_impl(toks: read toks, start: read (pclose + 1), end: read sigEnd)
+    // A `fresh` anywhere in the return-type region (e.g. `-> Result<fresh String, E>`)
+    // also sets returns-fresh, matching parse_function_decl's region scan.
+    if hasRet && fresh_in_return(toks: read toks, start: read retStart, sigEnd: read sigEnd) {
+        retFresh = true
+    }
     // Emit fn line.
     local sb = StringBuilder.new()
     StringBuilder.push(builder: mut sb, value: read "fn name=")
@@ -1532,6 +1549,20 @@ fn same_stmt_line(toks: read List<Tok>, a: read Int, b: read Int) -> Bool {
     return tk_line(toks: read toks, i: read a) == tk_line(toks: read toks, i: read b)
 }
 
+// True if `fresh` appears in the return-type region [start, sigEnd) before the
+// effects clause / body `{` / default-impl `=` (mirrors parse_function_decl).
+fn fresh_in_return(toks: read List<Tok>, start: read Int, sigEnd: read Int) -> Bool {
+    let mut k = start
+    while k < sigEnd {
+        if is_kw_text(toks: read toks, i: read k, word: read "effects") || at_symbol(toks: read toks, i: read k, code: read SYM_LBRACE) || at_symbol(toks: read toks, i: read k, code: read SYM_EQ) {
+            return false
+        }
+        if at_ident(toks: read toks, i: read k, wid: read WORD_FRESH) { return true }
+        k = k + 1
+    }
+    return false
+}
+
 // Index of a top-level `effects(` keyword in [start, end), or -1.
 fn find_effects_kw(toks: read List<Tok>, start: read Int, end: read Int) -> Int {
     let mut depth = 0

selfhost/samples/ast/async_let.rss

@@ -0,0 +1,5 @@
+async fn run(body: read String) -> Int {
+    async let a = Upload.put(path: read "/x", body: read body)
+    async let b = Upload.put(path: read "/y", body: read body)
+    return 0
+}

selfhost/samples/ast/nested_fresh.rss

@@ -0,0 +1,7 @@
+fn edition_of(package: read JsonValue) -> Result<fresh String, JsonError> {
+    return Ok(value: read "x")
+}
+
+fn path_array(m: read JsonValue) -> Result<fresh List<String>, JsonError> {
+    return Ok(value: read m)
+}