ISLE-native runtime migration: approved C1 design

This migration is an approved design, not a completed runtime rewrite. Its first milestone is C1: a small primitive-handler vertical that can register language-owned handlers while preserving the existing runtime manifest authority chain.

The proposed boundary uses ISLE to emit verifiable, stock Cranelift CLIF for a minimal primitive catalog. General HIR-to-CLIF lowering remains hand-written Rust. C1 is limited to primitive-handler work such as byte comparison and string equality; Rust fallbacks remain available during the staged rollout.

The design explicitly excludes replacing general expression lowering, porting frozen kernel exports, and porting fibers, channels, GC, composition, syscalls, or host-owned operations in this milestone. It also does not claim self-hosted handler compilation or a Rust-free toolchain.

The rollout principles are intentionally narrow: keep CLI, editor, and runtime versions aligned; rebuild artifacts after changing the contract; and expect version mismatches to be rejected. The relevant normative anchors remain the Platform Spec’s ABI and host material and runtime registration authority.

Provenance

Approved ISLE runtime-port design · ABI builtins authority · runtime registration authority