compiler+runtime: codegen fixes for empty literal, == int idents, m.field; runtime body-loss fix and Linux feature macros

Three codegen bugs surfaced repeatedly across the parallel port-to-El
agents and were patched here:

1. Empty array literal '[]' was emitting el_list_new(0, ) — trailing
   comma in a varargs call, fails the C parse. Special-cased: n==0
   returns 'el_list_empty()' directly.

2. '==' between two identifiers both tracked in __int_names (typed
   Int via 'let x: Int = ...') was miscompiling to str_eq. With the
   tagged-pointer Int-as-int64 representation, str_eq strcmp's what
   are integer values dressed as char* and segfaults on the first
   non-printable byte. Added the int-name lookup, mirroring the
   dispatch already present for '+' between Int idents. NotEq got
   the same treatment.

3. 'm.field' codegen was passing the raw const char* field name to
   el_get_field, which expects el_val_t. C compiler warned about int
   conversion; runtime read garbage at the address. Wrapped in
   EL_STR(...) so the field name lands as a proper el_val_t.

Runtime additions in the same pass:

  - el_runtime.c http_read_request: the loop's boundary check was
    'line_end >= hdr_end' which broke before processing the LAST
    header line — its trailing \r\n IS hdr_end. Real curl clients
    put Content-Length last, so POST bodies were silently arriving
    as length 0. Changed to '> hdr_end' so the last line is processed.
    soma-server agent surfaced this during smoke testing.

  - _GNU_SOURCE feature macro: clock_gettime/CLOCK_REALTIME, strcasecmp,
    and the dlfcn extensions (RTLD_DEFAULT) all gated behind it on
    glibc/Debian. macOS is permissive without; the landing Docker
    build needed these for linux/amd64. Adds <strings.h> for
    strcasecmp.

  - Refactored slot semantics in el_runtime.c (already in tree from
    the morning ARC commit): magic-tagged ElHeader at offset 0,
    ElList/ElMap with separate elems/keys/values payload allocations,
    el_list_append and el_map_set mutate-in-place when refcount<=1
    and copy-on-write when shared.

Self-host fixpoint reached at v3: elc → elc.c → cc → elc binary →
elc.c reproduced byte-for-byte. dist/platform/elc and dist/platform/elc.c
updated. The codegen.el and elc-combined.el changes are mirror-edits;
both flow through the bootstrap chain to keep self-hosting clean.

elc-combined.el

@@ -1703,9 +1703,11 @@ fn cg_expr(expr: Map<String, Any>) -> String {
         }
 
         // String equality: use str_eq() when either side is a string literal or ident.
-        // Use plain == when comparing integer literals.
+        // Use plain == when comparing integer literals OR when both sides are
+        // identifiers tracked in __int_names. Without the int-name check,
+        // `seen == idx` between two Int locals miscompiles to str_eq on what
+        // are integer values dressed as char* — segfault.
         if op == "EqEq" {
-            // Integer literal on either side → arithmetic comparison
             if left_kind == "Int" {
                 return "(" + left_c + " == " + right_c + ")"
             }
@@ -1718,6 +1720,17 @@ fn cg_expr(expr: Map<String, Any>) -> String {
             if right_kind == "Bool" {
                 return "(" + left_c + " == " + right_c + ")"
             }
+            if left_kind == "Ident" {
+                if right_kind == "Ident" {
+                    let lname: String = left["name"]
+                    let rname: String = right["name"]
+                    if is_int_name(lname) {
+                        if is_int_name(rname) {
+                            return "(" + left_c + " == " + right_c + ")"
+                        }
+                    }
+                }
+            }
             if left_kind == "Str" {
                 return "str_eq(" + left_c + ", " + right_c + ")"
             }
@@ -1751,6 +1764,17 @@ fn cg_expr(expr: Map<String, Any>) -> String {
             if right_kind == "Bool" {
                 return "(" + left_c + " != " + right_c + ")"
             }
+            if left_kind == "Ident" {
+                if right_kind == "Ident" {
+                    let lname: String = left["name"]
+                    let rname: String = right["name"]
+                    if is_int_name(lname) {
+                        if is_int_name(rname) {
+                            return "(" + left_c + " != " + right_c + ")"
+                        }
+                    }
+                }
+            }
             if left_kind == "Str" {
                 return "!str_eq(" + left_c + ", " + right_c + ")"
             }
@@ -1822,7 +1846,7 @@ fn cg_expr(expr: Map<String, Any>) -> String {
         let obj = expr["object"]
         let field: String = expr["field"]
         let obj_c: String = cg_expr(obj)
-        return "el_get_field(" + obj_c + ", " + c_str_lit(field) + ")"
+        return "el_get_field(" + obj_c + ", EL_STR(" + c_str_lit(field) + "))"
     }
 
     if kind == "Index" {
@@ -1845,6 +1869,7 @@ fn cg_expr(expr: Map<String, Any>) -> String {
     if kind == "Array" {
         let elems = expr["elems"]
         let n: Int = native_list_len(elems)
+        if n == 0 { return "el_list_empty()" }
         let items = ""
         let i = 0
         while i < n {