erlang: exit-signal propagation + trap_exit (+11 tests)

plans/erlang-on-sx.md

@@ -80,7 +80,7 @@ Core mapping:
 
 ### Phase 4 — links, monitors, exit signals
 - [x] `link/1`, `unlink/1`, `monitor/2`, `demonitor/1` — **17 new eval tests**; `make_ref/0`, `is_reference/1`, refs in `=:=`/format wired
-- [ ] Exit-signal propagation; trap_exit flag
+- [x] Exit-signal propagation; trap_exit flag — **11 new eval tests**; `process_flag/2`, monitor `{'DOWN', ...}`, `{'EXIT', From, Reason}` for trap-exit links, cascade death without trap_exit
 - [ ] `try/catch/of/end`
 
 ### Phase 5 — modules + OTP-lite
@@ -99,6 +99,7 @@ Core mapping:
 
 _Newest first._
 
+- **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.
 - **2026-04-25 link/unlink/monitor/demonitor + refs green** — Refs added to scheduler (`:next-ref`, `er-ref-new!`); `er-mk-ref`, `er-ref?`, `er-ref-equal?` in runtime. Process record gains `:monitored-by`. New BIFs in `lib/erlang/runtime.sx`: `make_ref/0`, `is_reference/1`, `link/1` (bidirectional, no-op for self, raises `noproc` for missing target), `unlink/1` (removes both sides; tolerates missing target), `monitor(process, Pid)` (returns fresh ref, adds entries to monitor's `:monitors` and target's `:monitored-by`), `demonitor(Ref)` (purges both sides). Refs participate in `er-equal?` (id compare) and render as `#Ref<N>`. 17 new eval tests covering `make_ref` distinctness, link return values, bidirectional link recording, unlink clearing both sides, monitor recording both sides, demonitor purging. Total suite 375/375. Signal propagation (the next checkbox) will hook into these data structures.
 - **2026-04-25 ring benchmark recorded — Phase 3 closed** — `lib/erlang/bench_ring.sh` runs the ring at N ∈ {10, 50, 100, 500, 1000} and times each end-to-end via wall clock. `lib/erlang/bench_ring_results.md` captures the table. Throughput plateaus at ~30-34 hops/s. 1M-process target IS NOT MET in this architecture — extrapolation = ~9h. The sub-task is ticked as complete with that fact recorded inline because the perf gap is architectural (env-copy per call, call/cc per receive, mailbox rebuild on delete-at) and out of scope for this loop's iterations. Phase 3 done; Phase 4 (links, monitors, exit signals, try/catch) is next.
 - **2026-04-25 conformance harness + scoreboard green** — `lib/erlang/conformance.sh` loads every test suite via the epoch protocol, parses pass/total per suite via the `(N M)` lists, sums to a grand total, and writes both `lib/erlang/scoreboard.json` (machine-readable) and `lib/erlang/scoreboard.md` (Markdown table with ✅/❌ markers). 9 suites × full pass = 358/358. Exits non-zero on any failure. `bash lib/erlang/conformance.sh -v` prints per-suite counts. Phase 3's only remaining checkbox is the 1M-process ring benchmark target.