fix(codegen): drop branch-consume-split owned params (#63 leg 3)

The last leg of the #63 drop-soundness cluster: an (own ...) heap param
consumed on one match-arm / if-branch but live on a sibling was never
dropped on the live path, leaking one slab per call. This was the
residual live=2 leak in series_sma that kept the series headline from
being leak-clean.

Root: the per-fn aggregate consume_count (the worst-case max over all
branches from uniqueness::merge_states) was codegen's only consume
signal, and every owned-param drop site gated on it. A param consumed
on SOME branch makes the aggregate >= 1, so every site skipped it —
including the branch where it stays live. match and if share this root
(if is first-class MTerm::If, not desugared to match).

Mechanism (spec 0068):
- CAPTURE: uniqueness retains the per-branch consume snapshots it
  already computes per branch and discarded at the max merge — new
  BranchConsume + infer_module_with_cross_branches (the aggregate-only
  infer_module_with_cross now delegates and drops the channel).
- CARRY: additive MArm.consume / MTerm::If.{then,else}_consume, attached
  by lower_to_mir via a traversal-order cursor (post-order pop, kind- and
  exhaustion-asserts make a desync a loud panic; neutral for const bodies
  which uniqueness does not walk). The AST carries no node id, so the
  correspondence is the structural pre-order both walks share — pinned by
  branch_consume_maps_attach_to_matching_arms.
- GATE: emit_leakclass_branch_param_drops fires a fall-through drop only
  for the leak class (branch_consume==0 AND aggregate>=1), in match arms
  + both if branches; the pre-tail-call dec switches its gate source from
  the aggregate to per-arm. The fn-return dec and arm-close pattern-binder
  dec are UNCHANGED.

Safety:
- Double-free: the drop sites partition by aggregate (==0 -> existing
  fn-return/pre-tail-call; >=1 -> new per-branch). Disjoint, so no param
  is dropped twice — no fn-return disable, no tail-position analysis.
- Use-after-free: the checker rejects use-after-consume, so an
  aggregate>=1 param is provably dead past the construct (path-terminal
  drop, tail position irrelevant).
- Type eligibility (found by the full-suite gate during implement; spec
  0068 refined): the new agg>=1 site is the FIRST drop site that can
  reach a STATIC closure-pair param (a top-level fn ref like inc in
  Either.either, consumed on one arm, live on the other) — a .rodata
  constant with no rc-header. field_drop_call routes Type::Fn / static-Str
  / Type::Var to the bare ailang_rc_dec, which underflowed on it. The
  helper now drops only params with a real per-type heap-ADT drop fn
  (field_drop_call != "ailang_rc_dec"). Sound (no underflow) and
  leak-correct (a static closure/Str allocates nothing). Witnessed green
  by std_either_demo / std_list_demo / poly_rec_capture_demo.

Verification: full workspace suite green (116 binaries); both new leak
pins (if + match) and series_sma_no_leak_pin green at live=0; legs 1/2
pins still green; INTERCEPTS<->(intrinsic) bijection intact; no
Pattern::Lit reject path added.

The leg-3 helper's type precondition was applied inline by the
orchestrator (a narrowing guard clause in an already-reviewed Task-4
helper, full context loaded) after the implement-orchestrator correctly
surfaced the spec gap rather than papering over the regression.

This clears the series_sma leak tail; the series milestone (#61) close
stays a separate deliberate step (its end-to-end milestone fieldtest).

closes #63

crates/ail/tests/series_sma_no_leak_pin.rs

@@ -0,0 +1,78 @@
+//! Pins that the #61 headline `series_sma` example is leak-clean under
+//! `--alloc=rc`. The residual `live=2` was #63 leg 3 (branch-consume-
+//! split); this pin closes that leak tail.
+//!
+//! Property protected: `series_sma` runs to completion under
+//! `AILANG_RC_STATS=1` with `live == 0` (equivalently `allocs == frees`).
+//! `series_sma_pin.rs` already pins stdout; this pin is the RC ledger,
+//! distinct. Goes green once the branch-consume-split leak is fixed.
+
+use std::path::Path;
+use std::process::Command;
+
+fn ail_bin() -> &'static str {
+    env!("CARGO_BIN_EXE_ail")
+}
+
+#[test]
+fn series_sma_is_leak_clean() {
+    let manifest_dir = env!("CARGO_MANIFEST_DIR");
+    let workspace = Path::new(manifest_dir).parent().unwrap().parent().unwrap();
+    let src = workspace.join("examples").join("series_sma.ail");
+    let tmp = std::env::temp_dir().join(format!(
+        "ailang_series_sma_no_leak_pin_{}",
+        std::process::id()
+    ));
+    std::fs::create_dir_all(&tmp).unwrap();
+    let out = tmp.join("bin");
+    let status = Command::new(ail_bin())
+        .args(["build", src.to_str().unwrap(), "--alloc=rc", "-o"])
+        .arg(&out)
+        .status()
+        .expect("ail build failed to run");
+    assert!(
+        status.success(),
+        "ail build --alloc=rc failed for series_sma.ail"
+    );
+    let output = Command::new(&out)
+        .env("AILANG_RC_STATS", "1")
+        .output()
+        .expect("execute binary");
+    assert!(
+        output.status.success(),
+        "binary exited non-zero: status {:?}",
+        output.status
+    );
+    let stderr = String::from_utf8(output.stderr).expect("stderr utf8");
+    let stats_line = stderr
+        .lines()
+        .find(|l| l.starts_with("ailang_rc_stats:"))
+        .unwrap_or_else(|| {
+            panic!("missing ailang_rc_stats line in stderr; stderr was:\n{stderr}")
+        });
+    let mut allocs: Option<u64> = None;
+    let mut frees: Option<u64> = None;
+    let mut live: Option<i64> = None;
+    for tok in stats_line.split_whitespace() {
+        if let Some(v) = tok.strip_prefix("allocs=") {
+            allocs = v.parse().ok();
+        } else if let Some(v) = tok.strip_prefix("frees=") {
+            frees = v.parse().ok();
+        } else if let Some(v) = tok.strip_prefix("live=") {
+            live = v.parse().ok();
+        }
+    }
+    let allocs = allocs.expect("missing allocs= field");
+    let frees = frees.expect("missing frees= field");
+    let live = live.expect("missing live= field");
+    assert_eq!(
+        live, 0,
+        "series_sma leaks {live} slab(s) (allocs={allocs} frees={frees}): the \
+         #61 residual live=2 was the #63 leg-3 branch-consume-split leak; if \
+         this is non-zero the leak-class drop is not firing on the series path."
+    );
+    assert_eq!(
+        allocs, frees,
+        "alloc/free mismatch (allocs={allocs} frees={frees} live={live})"
+    );
+}