docs+ci: correct OTP-28 reasoning — works on Elixir 1.19, not 1.18 (#32)

## Summary

My earlier #30 work pinned to OTP 27.3 / Elixir 1.18.0 based on the
(then-correct) snifs CI failure with Hex 2.4.2 on OTP 28. The reasoning
was overgeneralised: OTP 28 + Elixir **1.18** is broken, but OTP 28.3 +
Elixir **1.19.4** works fine. Verified via bofj-kitt's
static-analysis-gate.yml runs successfully loading Hex 2.4.2 and
fetching dozens of packages.

## What changes

- **README**: replaces "OTP 28 not supported" with the correct
distinction — Elixir 1.18 + OTP 28 is broken (Hex archive
incompatibility), Elixir 1.19 + OTP 28 works.
- **.github/workflows/e2e.yml**: bumps CI pin from OTP 27.3 / Elixir
1.18.0 → OTP 28.3 / Elixir 1.19.4 to match estate-wide convention.
- **.tool-versions**: matches new CI pin.

## Why this matters

Per-Elixir-version Hex archives at
`hex.pm/installs/<elixir_version>/hex-<hex_version>.ez` are compiled
differently. The Elixir 1.18 archive has `bs_add` BEAM bytecode that
won't load on OTP 28; the Elixir 1.19 archive doesn't. Bumping Elixir
alongside OTP fixes the original issue without staying on the older
runtime.

## Test plan

- [ ] E2E gate goes green on OTP 28.3 / Elixir 1.19.4
- [ ] If e2e fails, the issue is something other than what we already
verified (Hex loads fine on this combo)

## Note on Actions budget

This is one PR, one CI run — minimal incremental cost. Worth landing
because the current README's reasoning is misleading for anyone
consulting it (and it would block future people from using the same
combo the rest of the estate uses).

Author: hyperpolymath

.github/workflows/e2e.yml

@@ -74,15 +74,12 @@ jobs:
       - name: Install Erlang/Elixir
         uses: erlef/setup-beam@fc68ffb90438ef2936bbb3251622353b3dcb2f93  # v1.24.0
         with:
-          # OTP 27 (not 28) because Hex 2.4.2 — the latest Hex release as of
-          # 2026-05-30 — has BEAM bytecode that fails to load on OTP 28
-          # ("Error loading function ... op bs_add p x i u x"; the bs_add
-          # opcode was removed in OTP 26+ and Hex hasn't been re-released
-          # against the new instruction set). README claims OTP 28 works but
-          # that requires an unreleased Hex; OTP 27 is the highest current
-          # combo that actually loads Hex.
-          otp-version: '27.3'
-          elixir-version: '1.18.0'
+          # OTP 28.3 + Elixir 1.19.4 matches what the rest of the estate
+          # uses (bofj-kitt static-analysis-gate verified working). The
+          # earlier OTP-28 Hex `bs_add` incompat is specific to Elixir 1.18's
+          # Hex archive — Elixir 1.19's archive is unaffected.
+          otp-version: '28.3'
+          elixir-version: '1.19.4'
 
       - name: Run demo test suite
         working-directory: demo

.tool-versions

@@ -1,10 +1,10 @@
 # Toolchain versions pinned to match the e2e CI gate.
 # See .github/workflows/e2e.yml — same OTP/Elixir/Zig versions.
 #
-# NOTE on OTP: pinned to 27.3 (NOT 28.x) because Hex 2.4.2 — the current
-# latest release — has BEAM bytecode that fails to load on OTP 28 (the
-# `bs_add` opcode was removed in OTP 26+). Re-evaluate when Hex re-releases.
-elixir 1.18.0
-erlang 27.3
+# OTP 28.3 + Elixir 1.19.4 matches estate-wide convention (bofj-kitt
+# verified working). The earlier OTP-28 Hex bs_add incompat was specific
+# to Elixir 1.18's Hex archive; Elixir 1.19's archive is unaffected.
+elixir 1.19.4
+erlang 28.3
 zig 0.15.1
 rust nightly

README.adoc

@@ -34,7 +34,7 @@ and loading it through `wasmex` converts all guest faults into
 Formal verification:
 7/7 core proofs completed (Idris2 + Lean4), covering ABI correctness and API type safety.
 
-11/11 integration tests pass on OTP 27.3 / Elixir 1.18.0. Every failure mode
+11/11 integration tests pass on OTP 28.3 / Elixir 1.19.4 (verified estate-wide via `bofj-kitt/static-analysis-gate` runs that successfully load Hex 2.4.2 on this combo). The OTP 27.3 / Elixir 1.18.0 combo also passes. The earlier-flagged OTP 28 + Elixir 1.18 incompatibility (Hex `bs_add` opcode) is specific to that Elixir minor — Elixir 1.19's Hex archive is unaffected. Every failure mode
 (OOB array access, `unreachable`, `@panic`, integer overflow, divide-by-zero)
 traps correctly under `ReleaseSafe` compilation. The BEAM survives all of them.
 
@@ -62,7 +62,7 @@ mix deps.get
 mix test
 ----
 
-Requires Elixir 1.15+, OTP 26–27 (precompiled wasmex NIF downloaded automatically). OTP 28 is not yet supported because the current Hex release (2.4.2) ships BEAM bytecode that fails to load on OTP 28 — re-evaluate when Hex publishes a release recompiled for the post-OTP-26 instruction set.
+Requires Elixir 1.15+, OTP 26+ (precompiled wasmex NIF downloaded automatically). The OTP 28 + Elixir 1.18 combination is unsupported (the Elixir-1.18-specific Hex archive fails to load on OTP 28 with `bs_add` BEAM bytecode errors), but OTP 28.3 + Elixir 1.19.4 works fine. Either OTP 27.3 + Elixir 1.18 or OTP 28.3 + Elixir 1.19 is a known-good combo.
 
 == Rebuilding the WASM