erlang: make BEAM runtime smoke self-contained (real Node smoke)

[brandonpayton]

What

Make the Erlang/OTP 28 BEAM runtime smoke a real Node smoke instead of
artifact-only. The suite packages/registry/erlang/test/erlang.test.ts already
booted BEAM on Kandelo for 4 cases, but its 5th test called an external ring
module that is never compiled, so ring:start() died with
{undef,[{ring,start,...}]}.

This replaces the ring test with a self-contained named-fun ring driven via
-eval, so all 5 tests boot BEAM and run a real Erlang program with no
compilation step:

• prints hello world, evaluates arithmetic, handles lists and pattern matching, spawns lightweight processes, and runs a ring benchmark with inter-process message passing (spawn/2 + message passing across a 50-process
  ring, sized so the ~0.06s compute finishes well inside serve.ts's 2s
  idle-exit watchdog).
• demo/ring.erl and demo/serve.ts doc comments are corrected: they had
  advertised ring:start()., which cannot work on-platform.

Why not just compile ring.erl on Kandelo?

The on-Kandelo Erlang compiler crashes in beam_asm because erlang:md5/1
returns badarg on iolist input in this wasm BEAM build (it works only on a
binary), and beam_asm computes each module's MD5 over an iolist of chunks.
Verified directly: erlang:md5(<<"test">>) → <<9,143,...>>; erlang:md5("abc")
→ error:badarg. Filed as follow-up kd-qe2c. Until that is fixed, Erlang
modules must be precompiled on a host; the smoke therefore uses a self-contained
-eval program that needs no compilation.

Verification

• Node smoke: cd host && npx vitest run ../packages/registry/erlang/test/erlang.test.ts
  inside scripts/dev-shell.sh → 5 passed / 5. Outcome lists + JSON report
  under test-runs/kd-fyqz/.
• maxAddr: explicit and logged by serve.ts — max_addr = 0x3fdc0000 (page
  16348), reserving the top of the wasm32 address space for BEAM thread
  channels/TLS.
• Browser smoke: skipped — no browser BEAM harness exists (the erlang demo
  has only a Node launcher). Documented as a boundary; a browser BEAM smoke is a
  candidate follow-up once the OTP runtime tree is a publishable sidecar
  (kd-1mr.6 / kd-yuef).
• ABI: scripts/check-abi-version.sh → snapshot/ABI_VERSION consistent (no
  kernel/ABI change here).
• Scope: this is a test + demo-doc change; it touches no Rust/kernel/libc/syscall
  code, so those gates are not relevant.

Notes / limitations

• The suite keeps its pre-existing describe.skipIf(!hasErlang) contract: it
  passes when erlang.wasm + the erlang-install/ OTP tree are present locally,
  and skips otherwise. The abi release ships only erlang.wasm, not the OTP
  tree, so CI/browser without a staged tree records these as skipped, not failed.
  Publishing the OTP runtime tree is separate packaging work (kd-1mr.6 / kd-yuef).
• The Homebrew erlang.rb Formula + build-erlang.sh sandbox changes from the
  kd-xhdd harvest are intentionally not included — they are build/Formula
  concerns that overlap a divergent erlang.rb already on kd-p3hr's Add Homebrew formulas for language runtimes #810, and
  belong to the build/publish lane, not this smoke harness.

Bead: kd-fyqz. Follow-up: kd-qe2c (erlang:md5/1 iolist badarg).

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