erlang: bank.erl account server (+8 tests)

plans/erlang-on-sx.md

@@ -72,7 +72,7 @@ Core mapping:
 - [ ] Classic programs in `lib/erlang/tests/programs/`:
   - [x] `ring.erl` — N processes in a ring, pass a token around M times — **4 ring tests**; suspension machinery rewritten from `shift`/`reset` to `call/cc` + `raise`/`guard`
   - [x] `ping_pong.erl` — two processes exchanging messages — **4 ping-pong tests**
-  - [ ] `bank.erl` — account server (deposit/withdraw/balance)
+  - [x] `bank.erl` — account server (deposit/withdraw/balance) — **8 bank tests**
   - [ ] `echo.erl` — minimal server
   - [ ] `fib_server.erl` — compute fib on request
 - [ ] `lib/erlang/conformance.sh` + runner, `scoreboard.json` + `scoreboard.md`
@@ -99,6 +99,7 @@ Core mapping:
 
 _Newest first._
 
+- **2026-04-24 bank.erl green** — `lib/erlang/tests/programs/bank.sx` with 8 tests. Stateful server pattern: `Server = fun (Balance) -> receive ... Server(NewBalance) end end` recursively threads balance through each iteration. Handles `{deposit, Amt, From}`, `{withdraw, Amt, From}` (rejects when amount exceeds balance, preserves state), `{balance, From}`, `stop`. Tests cover deposit accumulation, withdrawal within balance, insufficient funds with state preservation, mixed transactions, clean shutdown, two-client interleave. Total suite 343/343.
 - **2026-04-24 ping_pong.erl green** — `lib/erlang/tests/programs/ping_pong.sx` with 4 tests: classic Pong server + Ping client with separate `ping_done`/`pong_done` notifications, 5-round trace via io-buffer (`"ppppp"`), main-as-pinger-4-rounds (no intermediate Ping proc), tagged-id round-trip (`"4 3 2 1 "`). All driven by `Ping = fun (Target, K) -> ... Ping(Target, K-1) ... end` self-recursion — captured-env reference works because `Ping` binds in main's mutable env before any spawned body looks it up. Total suite 335/335.
 - **2026-04-24 ring.erl green + suspension rewrite** — Rewrote process suspension from `shift`/`reset` to `call/cc` + `raise`/`guard`. **Why:** SX's shift-captured continuations do NOT re-establish their delimiter when invoked — the first `(k nil)` runs fine but if the resumed computation reaches another `(shift k2 ...)` it raises "shift without enclosing reset". Ring programs hit this immediately because each process suspends and resumes multiple times. `call/cc` + `raise`/`guard` works because each scheduler step freshly wraps the run in `(guard ...)`, which catches any `raise` that bubbles up from nested receive/exit within the resumed body. Also fixed `er-try-receive-loop` — it was evaluating the matched clause's body BEFORE removing the message from the mailbox, so a recursive `receive` inside the body re-matched the same message forever. Added `lib/erlang/tests/programs/ring.sx` with 4 tests (N=3 M=6, N=2 M=4, N=1 M=5 self-loop, N=3 M=9 hop-count via io-buffer). All process-communication eval tests still pass. Total suite 331/331.
 - **2026-04-24 exit/1 + termination green** — `exit/1` BIF uses `(shift k ...)` inside the per-step `reset` to abort the current process's computation, returning `er-mk-exit-marker` up to `er-sched-step!`. Step handler records `:exit-reason`, clears `:exit-result`, marks dead. Normal fall-off-end still records reason `normal`. `exit/2` errors with "deferred to Phase 4 (links)". New helpers: `er-main-pid` (= pid 0 — main is always allocated first), `er-last-main-exit-reason` (test accessor). 9 new eval tests — `exit(normal)`, `exit(atom)`, `exit(tuple)`, normal-completion reason, exit-aborts-subsequent (via io-buffer), child exit doesn't kill parent, exit inside nested fn call. Total eval 174/174; suite 327/327.