v0.4.8

Added

• Host Extension API (prd-host-extension-api.md Phases 1–3), behind
  the new CEDAR_HOST_EXTENSIONS CMake option — default OFF, so the
  WASM/web build, the CLI and every existing program are untouched.
  Embedders can register, once at init:
  • akkado::register_host_variable(...) — control-rate host signals,
    desugared onto the existing bpm/sr ENV_GET path. Drive them via
    vm.set_param(akkado::host_variable_env_key("name"), x).
  • akkado::register_host_function(...) — with a null impl_fn it
    aliases an existing core opcode; with an impl_fn it becomes a real
    host opcode.
  • akkado::register_host_node(...) — a stateful node naming a
    heavyweight host-owned instance (nkido studio's plugin("Diva")).
    Codegen records each call site in CompileResult::required_host_nodes
    so the host can bind a pooled instance to that state_id off the
    audio thread. The engine never loads, prepares or destroys the
    instance, and needs no lifecycle hooks.
• HOST_OP opcode (209) dispatched through cedar::HostOpRegistry,
  a 256-entry table indexed by inst.rate. Host extensibility costs one
  enum slot regardless of how many host ops are registered; core opcodes
  keep their jump table and only host ops pay one indirect call.
• HostOpState — arena-backed per-instance host-op state with a
  release() hook, so the existing HasArenaRelease StatePool sweep
  reclaims it. No per-region arena free had to be added.

Name collisions with a core builtin, alias, or variable — or with an
earlier host registration — are rejected, never shadowed. Audio-rate host
variables and hosted-node event-stream inputs are likewise rejected
rather than silently ignored; both land with Phase 4 and the studio's
plugin backend respectively.

Nondestructiveness verified: full cedar + akkado suites pass with the flag
both ON and OFF, registering a host op leaves an unrelated program's
bytecode byte-identical, and three fixtures rendered 300 s each produce
bit-identical WAVs from ON and OFF builds.