fix(zset): mark a rebuild chunk's push as a step so a panicking push is counted (improvise-bw4p.5)

Also renames the MemTrace clone-panic test to what it asserts.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VdfEGXA7aJ6ftMsCc9PJze

context/test-evidence/zset-ffi.md

@@ -215,3 +215,9 @@ A job whose first chunk failed inside the transaction resumed without that chunk
 ### f3_begin_on_a_circuit_poisoned_outside_a_txn_is_poisoned
 
 `begin` is `ZS_ERR_POISONED`; `commit` and `rollback` are `ZS_ERR_TRANSACTION`.
+
+### f5_a_chunk_whose_push_overflows_in_a_txn_then_rollback_is_interrupted
+
+The same path through the ABI: `zs_rebuild_step` returns `ZS_ERR_OVERFLOW` from the chunk's
+push, `zs_circuit_rollback` succeeds, and the next `zs_rebuild_step` was a silent success; it is
+now `ZS_ERR_INTERRUPTED`. Rival: the mark before the push removed fails `[f5]`.

context/test-evidence/zset.md

@@ -566,7 +566,7 @@ key is only visible when it is touched again (a rival that mutated `DistinctStat
 cloning its entry survived the first version). Rivals, each failing it: `add_logged`,
 `DistinctState::step` and `Undoable::apply` each mutating before cloning the entry.
 
-### c2_memtrace_value_clone_panic_leaves_an_empty_group
+### a_panicking_value_clone_leaves_a_memtrace_unchanged
 
 `MemTrace::apply` inserted an empty group before cloning the value. Rival: that order restored
 fails `[c2]`.
@@ -589,3 +589,10 @@ split the tick inside the transaction and never reached the save).
 Pins the limit `zset.md` states: a first continued piece in a transaction clones the edge's
 whole value, growing with the state.
 
+
+### p1_a_chunk_whose_push_panics_in_a_txn_is_counted
+
+Delta review of #355 at b2445fed. `Rebuild::step` takes the chunk from its iterator and pushes
+it before stepping; a push that overflows the pending delta panicked with no step run, so
+`undone` was not bumped and the resumed job silently dropped the chunk (key 1's `+1`). The step
+is now marked before the push. Rival: that mark removed fails `[p1]`.