runtime: ARC scaffolding + indirection so el_list_append amortizes O(1)

The compiler used to OOM at ~8.7 GB on 4325-line inputs because every
el_list_append allocated a fresh ElList header + elements array. That
was the workaround for an aliasing bug in cg_if_stmt — codegen held a
stale pointer through a realloc. Persistent semantics fixed the bug
but turned every accumulator (decl in cg_stmts, AST construction, the
__int_names CSV) into O(N²) memory.

Real fix in two coordinated parts:

1. Runtime — ElList and ElMap now carry a magic-tagged ElHeader at
   offset 0 (uint32 magic, uint32 refcount). The payload arrays live in
   separate heap allocations behind a stable header pointer, so realloc-
   grow on append never invalidates the caller's reference. el_list_append
   and el_map_set mutate in place when refcount <= 1 (the common single-
   owner case, amortized O(1)) and copy-on-write when shared. Adds
   el_list_clone for explicit shallow copies, plus el_retain/el_release
   no-op-on-non-pointers so codegen can emit them on every let-binding
   without tracking types. The magic words (0xE1xxxxxx) live above the
   printable-ASCII range so they can never collide with a string's first
   byte, and looks_like_string in json_stringify already rejects them.

2. Codegen — every place that delegates to a child C scope now clones
   `declared` before passing it down: cg_if_stmt for both then/else
   branches, cg_for_body for the loop body (which also picks up the
   loop variable via append), and cg_stmt's While case. Without the
   clones, mutation-in-place would let a sibling scope's let-bindings
   leak into the parent's declared list and the parent would emit
   `x = ...` against an undeclared name. The clones are cheap shallow
   copies of a list of strings.

Result on the landing-combined.el (4325 lines): 8.7 GB → 3.5 GB peak,
0.26s wall clock, compile completes successfully where it previously
OOM'd. Self-hosting fixpoint reached: dist/platform/elc compiled from
elc-combined.el reproduces dist/platform/elc.c byte-for-byte on a
second pass through itself.

Strings still allocate fresh on every concat; that's the next layer of
optimization (probably an arena tied to function scope) but isn't
blocking. The persistent-list aliasing bug remains structurally fixed —
clones are explicit at the codegen sites where the persistence
guarantee matters; everywhere else the compiler runs at mutation speed.

el-compiler/runtime/el_runtime.h

@@ -94,6 +94,20 @@ el_val_t  el_abs(el_val_t n);
 el_val_t  el_max(el_val_t a, el_val_t b);
 el_val_t  el_min(el_val_t a, el_val_t b);
 
+/* ── Refcount (ARC) ──────────────────────────────────────────────────────────
+ * Lists and Maps carry a refcount. Strings and ints do not — el_retain and
+ * el_release are safe no-ops on non-refcounted values (they sniff a magic
+ * header at offset 0 and only act if the magic matches).
+ *
+ * Codegen emits these at let-binding shadowing, function entry (params), and
+ * function exit (locals other than the returned value). The refcount lets
+ * el_list_append and el_map_set mutate in place when uniquely owned (cheap)
+ * and copy-on-write when shared (preserves persistent semantics across
+ * accumulator patterns in the compiler itself).                              */
+
+void      el_retain(el_val_t v);
+void      el_release(el_val_t v);
+
 /* ── List ────────────────────────────────────────────────────────────────── */
 
 el_val_t  el_list_new(el_val_t count, ...);
@@ -101,6 +115,7 @@ el_val_t  el_list_len(el_val_t list);
 el_val_t  el_list_get(el_val_t list, el_val_t index);
 el_val_t  el_list_append(el_val_t list, el_val_t elem);
 el_val_t  el_list_empty(void);
+el_val_t  el_list_clone(el_val_t list);
 
 /* ── Map ─────────────────────────────────────────────────────────────────── */
 
@@ -320,6 +335,7 @@ el_val_t  native_list_get(el_val_t list, el_val_t index);
 el_val_t  native_list_len(el_val_t list);
 el_val_t  native_list_append(el_val_t list, el_val_t elem);
 el_val_t  native_list_empty(void);
+el_val_t  native_list_clone(el_val_t list);
 el_val_t  native_string_chars(el_val_t s);
 el_val_t  native_int_to_str(el_val_t n);