Five-task plan for spec 0068, decomposed RED-first:
1. RED — new if-branch fixture + pin, un-ignore the committed match pin.
2. CAPTURE — retain per-branch consume snapshots in uniqueness (new
BranchConsume + infer_module_with_cross_branches; old API delegates).
3. CARRY — additive MArm.consume / MTerm::If.{then,else}_consume,
attached by a traversal-order cursor in lower_to_mir (post-order pop,
exhaustion assert, neutral for const bodies) + lock-step test.
4. GATE — emit_leakclass_branch_param_drops helper + :823 gate-source
switch to per-arm; fall-through drops in match arms + if branches.
Disjoint-by-aggregate from the unchanged fn-return dec.
5. VERIFY — series_sma live=0 pin, full workspace suite, lockspair check.
All inlined code lifted from the live source bodies (uniqueness.rs,
lower_to_mir.rs, match_lower.rs, lib.rs) read this session, not from
memory. The one new surface fixture (if-branch) parse-checks clean; the
correspondence test reuses the existing match fixture via the
lower_to_mir_ty.rs elaborate_fixture/body helpers (verified present).
refs #63
