host: doc — wasm kernel boot crash fixed (crypto), boost still blocked on web-stack load

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

plans/host-spa.md

@@ -43,7 +43,29 @@ graceful degradation to plain server-rendered pages with no JS.
   WASM kernel (sx.rose-ash.com avoids this because its Docker image ships a
   consistent bundle and it navigates via client-router page-routes, not boost).
 
-## Rebuild attempt (2026-06-28) — FAILED, reverted
+## UPDATE 2026-06-29 — kernel BOOT crash fixed (crypto WASM-safe)
+
+The boot crash was NOT the build pipeline — it was the kernel's crypto stack
+assuming 63-bit native int. On the web targets (js_of_ocaml 32-bit, wasm_of_ocaml
+31-bit) sha2/cbor/cid/ed25519 truncated, and ed25519 precomputes `sqrtm1` +
+`base_point` AT MODULE INIT via a base-2^26 bignum whose 52-bit products overflow
+→ `Char.chr(-4)` crash on load. Fixed in `fce9e0c6` (sx_sha2 Int32 rounds +
+Int64 length, sx_cbor Int64 width-select, sx_cid bounded base32, sx_ed25519 Int64
+bignum mul/div_small). Verified: NIST/CID vectors match native↔js↔wasm; native
+conformance 271/271; **the freshly-built browser kernel now BOOTS** (SxKernel
+live, data-sx-ready=true, crypto-sha256 correct on js + wasm).
+
+REMAINING for boost (separate layer — web-stack loading, NOT crypto):
+- `.sxbc` still fail `VM: unknown opcode 0` against the SX-level `vm.sx`
+  interpreter, even freshly recompiled — a bytecode-format mismatch (likely the
+  sx-vm-extensions merge changed sx_vm.ml's opcodes but the served `vm.sx` wasn't
+  regenerated). So the web stack falls back to `.sx` source.
+- `boot.sx` SOURCE then fails `Expected list, got string` (fails on the old
+  main-worktree kernel too), so `process-boosted` never runs → boost 0/N.
+  Next: either regenerate `vm.sx` to match the compiler so `.sxbc` load, or fix
+  the `boot.sx` source-eval failure so source fallback completes.
+
+## Rebuild attempt (2026-06-28) — FAILED, reverted (superseded by the fix above)
 
 Tried it: `dune build browser/sx_browser.bc.wasm.js` succeeded (with many
 `integer-overflow` warnings — "generated code might be incorrect"), and