fix: three foundation/el root-cause bugs (no more bandaids)

1. Parser+codegen: bare reassignment `x = expr` inside an if-body
   was compiling to three orphan expressions with no store. Now
   emits a real assignment.

2. Runtime json_get: dot-path segments that are all digits now
   correctly traverse array indices. `json_get(s, "0.field")` works.

3. Runtime HTTP writer: response bodies starting with
   `{"__status__":<int>,...}` now set the HTTP status header to
   that value and strip the marker from the served body. Existing
   404/401/503 paths in product code now produce real status codes
   instead of HTTP 200 with the status hidden in the body.

Self-host fixed point holds: gen2 == gen3 byte-identical.
Snapshot tagged at dist/platform/elc.20260502-1231-self-host.

Backlog: bl-c121edda

el-compiler/src/parser.el

@@ -946,6 +946,24 @@ fn parse_stmt(tokens: [Map<String, Any>], pos: Int) -> Map<String, Any> {
         }, p)
     }
 
+    // Bare reassignment: `name = expr`. Handled BEFORE the expression
+    // fallback so we don't drop the assign on the floor and emit three
+    // orphan expressions (the original silent-miscompile bug). El's `let`
+    // already permits redeclaration, so this only applies when the parser
+    // sees an Ident followed directly by `=`. `==` is a separate kind
+    // (EqEq) so there's no ambiguity.
+    if k == "Ident" {
+        let k2 = tok_kind(tokens, pos + 1)
+        if k2 == "Eq" {
+            let name = tok_value(tokens, pos)
+            let p = pos + 2
+            let r = parse_expr(tokens, p)
+            let val = r["node"]
+            let p = r["pos"]
+            return make_result({ "stmt": "Assign", "name": name, "value": val }, p)
+        }
+    }
+
     // bare expression or if/match statement
     let r = parse_expr(tokens, pos)
     let val = r["node"]