erlang: gen_server behaviour (+10 tests)

plans/erlang-on-sx.md

@@ -85,7 +85,7 @@ Core mapping:
 
 ### Phase 5 — modules + OTP-lite
 - [x] `-module(M).` loading, `M:F(...)` calls across modules — **10 new eval tests**; multi-arity, sibling calls, cross-module dispatch via `er-modules` registry
-- [ ] `gen_server` behaviour (the big OTP win)
+- [x] `gen_server` behaviour (the big OTP win) — **10 new eval tests**; counter + LIFO stack callback modules driven via `gen_server:start_link/call/cast/stop`
 - [ ] `supervisor` (simple one-for-one)
 - [ ] Registered processes: `register/2`, `whereis/1`
 
@@ -99,6 +99,7 @@ Core mapping:
 
 _Newest first._
 
+- **2026-04-25 gen_server (OTP-lite) green** — `er-gen-server-source` in `lib/erlang/runtime.sx` is the canonical Erlang text of the behaviour; `er-load-gen-server!` registers it in the user-module table. Implements `start_link/2`, `call/2` (sync via `make_ref` + selective `receive {Ref, Reply}`), `cast/2` (async fire-and-forget returning `ok`), `stop/1`, and the receive loop dispatching `{'$gen_call', {From, Ref}, Req}` → `Mod:handle_call/3`, `{'$gen_cast', Msg}` → `Mod:handle_cast/2`, anything else → `Mod:handle_info/2`. handle_call reply tuples supported: `{reply, R, S}`, `{noreply, S}`, `{stop, R, Reply, S}`. handle_cast/info: `{noreply, S}`, `{stop, R, S}`. `Mod:F` and `M:F` where `M` is a runtime variable now work via new `er-resolve-call-name` (was bug: passed unevaluated AST node `:value` to remote dispatch). 10 new eval tests: counter callback module (start/call/cast/stop, repeated state mutations), LIFO stack callback module (`{push, V}` cast, pop returns `{ok, V}` or `empty`, size). Total suite 425/425.
 - **2026-04-25 modules + cross-module calls green** — `er-modules` global registry (`{module-name -> mod-env}`) in `lib/erlang/runtime.sx`. `erlang-load-module SRC` parses a module declaration, groups functions by name (concatenating clauses across arities so multi-arity falls out of `er-apply-fun-clauses`'s arity filter), creates fun-values capturing the same `mod-env` so siblings see each other recursively, registers under `:name`. `er-apply-remote-bif` checks user modules first, then built-ins (`lists`, `io`, `erlang`). `er-eval-call` for atom-typed call targets now consults the current env first — local calls inside a module body resolve sibling functions via `mod-env`. Undefined cross-module call raises `error({undef, Mod, Fun})`. 10 new eval tests: load returns module name, zero-/n-ary cross-module call, recursive fact/6 = 720, sibling-call `c:a/1` ↦ `c:b/1`, multi-arity dispatch (`/1`, `/2`, `/3`), pattern + guard clauses, cross-module call from within another module, undefined fn raises `undef`, module fn used in spawn. Total suite 415/415.
 - **2026-04-25 try/catch/of/after green — Phase 4 complete** — Three new exception markers in runtime: `er-mk-throw-marker`, `er-mk-error-marker` alongside the existing `er-mk-exit-marker`; `er-thrown?`, `er-errored?` predicates. `throw/1` and `error/1` BIFs raise their respective markers. Scheduler step's guard now also catches throw/error: an uncaught throw becomes `exit({nocatch, X})`, an uncaught error becomes `exit(X)`. `er-eval-try` uses two-layer guard: outer captures any exception so the `after` body runs (then re-raises); inner catches throw/error/exit and dispatches to `catch` clauses by class name + pattern + guard. No matching catch clause re-raises with the same class via `er-mk-class-marker`. `of` clauses run on success; no-match raises `error({try_clause, V})`. 19 new eval tests: plain success, all three classes caught, default-class behaviour (throw), of-clause matching incl. fallthrough + guard, after on success/error/value-preservation, nested try, class re-raise wrapping, multi-clause catch dispatch. Total suite 405/405. **Phase 4 complete — Phase 5 (modules + OTP-lite) is next.** Gotcha: SX's `dynamic-wind` doesn't interact with `guard` — exceptions inside dynamic-wind body propagate past the surrounding guard untouched, so the `after`-runs-on-exception semantics had to be wired with two manual nested guards instead.
 - **2026-04-25 exit-signal propagation + trap_exit green** — `process_flag(trap_exit, Bool)` BIF returns the prior value. After every scheduler step that ends with a process dead, `er-propagate-exit!` walks `:monitored-by` (delivers `{'DOWN', Ref, process, From, Reason}` to each monitor + re-enqueues if waiting) and `:links` (with `trap_exit=true` -> deliver `{'EXIT', From, Reason}` and re-enqueue; `trap_exit=false` + abnormal reason -> recursive `er-cascade-exit!`; normal reason without trap_exit -> no signal). `er-sched-step!` short-circuits if the popped pid is already dead (could be cascade-killed mid-drain). 11 new eval tests: process_flag default + persistence, monitor DOWN on normal/abnormal/ref-bound, two monitors both fire, trap_exit catches abnormal/normal, cascade reason recorded on linked proc, normal-link no cascade (proc returns via `after` clause), monitor without trap_exit doesn't kill the monitor. Total suite 386/386. `kill`-as-special-reason and `exit/2` (signal to another) deferred.