go: panic + recover → eval 92/92, total 509/509, Phase 6 closed [shapes-scheduler]

Panic/recover builtins + per-frame __go-panic-cell of shape
(STATE V). Body panic flips cell :none→:raised BEFORE defers drain
so recover() can find it. recover() walks env chain past shadowing
cells to the outermost :raised one — flips it :recovered, returns V.
Frame exit checks cell: :recovered → return clean; :raised →
propagate (:go-panic V).

6 tests: uncaught-from-program, panic-from-fn, defer-recover-swallow,
recover-captures-via-channel, propagation-through-no-defer-chain,
middle-frame-catches-deeper-panic.

Shape: panic cell is a frame-attached out-of-band channel that
survives function boundaries via env-chain walk. Same primitive
slots into the scheduler kit's termination-record + cleanup-with-
error-context hook. Maps cleanly to Erlang try/catch/after.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

plans/go-on-sx.md

@@ -359,8 +359,12 @@ Progress-log line → push `origin/loops/go`.
 - [x] Defer stack per function frame; runs LIFO on normal return.
       Args eager at defer-time; frame-local (inner defers don't run
       outer ones); defer-in-loop pushes each iteration. 6 tests.
-- [ ] `panic(v)` unwinds frames running deferreds; `recover()` inside a
-      deferred fn captures the panic value and stops unwinding.
+- [x] `panic(v)` unwinds frames running deferreds; `recover()` inside a
+      deferred fn captures the panic value and stops unwinding. Panic
+      sentinel `(:go-panic V)` propagates through go-eval-block /
+      go-eval-stmt / go-eval-program-loop. Per-frame panic cell
+      `(STATE V)` flips :none → :raised → :recovered; recover walks
+      env chain finding the outermost :raised cell. 6 tests on eval/.
 - Goroutine panic propagation: a panicking goroutine that doesn't recover
   crashes the whole program (honour Go spec, or document divergence).
 - Tests: defer order (LIFO), defer + named-return mutation, panic/recover,
@@ -611,6 +615,24 @@ Minimal repro: see `lib/go/lex.sx#gl-oct-digit?` and `#gl-match-op`.
 
 _Newest first. Append one dated entry per commit._
 
+- 2026-05-27 — **Phase 6: panic + recover.** `panic` and `recover`
+  builtins. Panic sentinel `(:go-panic V)` propagates like
+  `:return-value` through stmt/block/program-loop. Each call frame
+  gets its own `__go-panic-cell` of shape `(STATE V)`; on body panic
+  the cell flips to `:raised V` BEFORE defers drain so `recover()`
+  can find it. `recover` walks the env chain looking for the
+  outermost `:raised` cell (so deferred calls invoked from a
+  panicking frame still see that frame's cell despite their own
+  nested cell shadowing it). Flips to `:recovered` and returns V;
+  the panicking frame then returns normally instead of propagating.
+  6 tests: uncaught surfaces from program, panic from fn surfaces,
+  defer-recover swallows, defer-recover captures value via channel,
+  propagation through 3-deep no-defer chain, middle frame catches
+  panic from deeper. 509/509 total. **Shape:** the panic cell is
+  a *frame-attached out-of-band channel* that survives across the
+  function boundary because env-chain lookup can walk past
+  shadowed bindings to find it. Same primitive will serve as the
+  scheduler kit's "cleanup-with-error-context" hook. [shapes-scheduler]
 - 2026-05-27 — **Phase 6 first slice: defer + LIFO.** Added
   `go-eval-defer-stmt`, `go-run-defers!`, `go-run-defers-prefix!`,
   plus new `:quoted-value` AST node so deferred calls can be