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

examples/http-status-envelope.el

@@ -0,0 +1,33 @@
+// http-status-envelope.el — acceptance test for the __status__ HTTP envelope.
+//
+// Before fix: a handler returning {"__status__":401,"error":"unauthorized"}
+// went out as an HTTP 200 with the JSON body verbatim, so Cloud Run logs were
+// full of 200s for what should have been 4xx/5xx.
+//
+// After fix: when the response body's FIRST key is __status__, the runtime
+// reads the integer value as the HTTP status code and strips the marker from
+// the body before sending it to the client.
+//
+// Verify with curl:
+//   curl -i http://localhost:8081/auth   -> HTTP/1.1 401 Unauthorized
+//   curl -i http://localhost:8081/health -> HTTP/1.1 200 OK
+//   curl -i http://localhost:8081/oops   -> HTTP/1.1 503 Service Unavailable
+
+fn handle(method: String, path: String, body: String) -> String {
+    if path == "/auth" {
+        return "{\"__status__\":401,\"error\":\"unauthorized\"}"
+    }
+    if path == "/oops" {
+        return "{\"__status__\":503,\"error\":\"degraded\"}"
+    }
+    if path == "/health" {
+        return "{\"ok\":true}"
+    }
+    return "{\"__status__\":404,\"error\":\"not found\"}"
+}
+
+fn main() -> Int {
+    http_set_handler("handle")
+    http_serve(8081, "handle")
+    return 0
+}

examples/json-array-traversal.el

@@ -0,0 +1,21 @@
+// json-array-traversal.el — acceptance test for json_get dot-path with array
+// indices.
+//
+// Before fix: json_get("...", "0.field") would substring-search for a literal
+// key named `"0.field"` and find nothing, returning "".
+//
+// After fix: dot-path segments that are all digits are treated as array
+// indices and the walker descends into the array.
+
+fn test_array_traversal() -> String {
+    let s: String = "[{\"name\":\"alice\"},{\"name\":\"bob\"}]"
+    let a: String = json_get(s, "0.name")
+    let b: String = json_get(s, "1.name")
+    return a + "," + b
+}
+
+fn main() -> Int {
+    let r: String = test_array_traversal()
+    print(r)
+    return 0
+}

examples/reassign-in-if.el

@@ -0,0 +1,22 @@
+// reassign-in-if.el — acceptance test for bare reassignment inside if-body.
+//
+// Before fix: parser dropped `x = "override"` on the floor and codegen emitted
+// three orphan expressions (`x; EL_NULL; EL_STR("override");`). Effective store
+// was lost, so the function returned "default".
+//
+// After fix: parse_stmt recognises `Ident "=" Expr` as an Assign statement and
+// codegen emits a real C assignment, so the function returns "override".
+
+fn test_reassign() -> String {
+    let x: String = "default"
+    if true {
+        x = "override"
+    }
+    return x
+}
+
+fn main() -> Int {
+    let r: String = test_reassign()
+    print(r)
+    return 0
+}