feat(routing-policy): native-Go preset engine so the TinyGo wasm-visor can run presets (no wazero)

[0pcom]

Problem

The built-in routing-policy presets only ran on the native visor. preset:<name> loads the embedded pkg/router/policy/wasm/presets/bundle.wasm and evaluates it via wazero. A wasm-visor is itself TinyGo wasm and cannot host wazero (wasm-in-wasm), so it never evaluated presets — cmd/wasm-visor built its router with a nil DialHook, and pkg/router/policy (+ .../wasm) don't compile under TinyGo (starlark / wazero).

Approach

Extract the preset decide/tick logic into one pure-Go package, pkg/router/policy/preset (stdlib-only: no unsafe, no JSON ABI, no wazero, no starlark) as the single source of truth, and compile it into both paths:

• docs/examples/routing-policies/wasm/bundle/main.go is now a thin wasm ABI shim over that package — it keeps only the JSON wire types and the alloc/free/decide_route/on_tick linear-memory glue, delegating every decision to preset. bundle.wasm is rebuilt from it (551798 → 551439 bytes). The native visor keeps running this bundle via wazero, unchanged.
• pkg/router/policy/presethook adapts the preset package to the router.DialHook / RouteSelectingHook / RotationHook interfaces without wazero — TinyGo-safe (imports only pkg/router + preset). cmd/wasm-visor wires it as the router's DialHook, selected by the ?routing_policy=<name> query param. Unset ⇒ nil hook ⇒ unchanged (no-policy) behavior; an unknown name is rejected (no silent fallback).

pkg/router/policy / hook.go are untouched — the native selection path (pkg/visor/policy_loader.go → wazero) is unchanged.

Byte-identical guarantee (gate-1, extended to the native path)

A new parity_test.go drives the same inputs through the wazero bundle and the native preset package and asserts identical results:

• decide: field-for-field for every preset (17 representative cases).
• on_tick: step-for-step over a multi-tick leg-snapshot sequence for each stateful controller (rotating-bw, latency-adaptive, elastic-mux, probe-and-prune, adaptive) — proving the EWMA smoothing, AIMD peak/idle counters, and probe state machine evolve identically across the two compilations.

The existing wazero end-to-end presets_test.go and the bundle's native main_test.go still pass unchanged.

Verified

• go build . (repo root)
• go test ./pkg/router/policy/... (incl. wazero end-to-end + new parity + new unit tests)
• GOOS=js GOARCH=wasm go build ./cmd/wasm-visor
• Full TinyGo-fork build of ./cmd/wasm-visor (the committed embed lane) — the preset engine compiles into the browser visor blob.

Follow-up (documented gap)

The wasm-visor now invokes the preset engine on every dial via the DialHook. The conditional presets geo-avoid / transport-diverse read per-hop geo / transport-kind metadata, supplied on native by a policy.Provider; on the wasm-visor this comes from a presethook.Provider, which is nil today (NopProvider), so those two defer until a wasm-side metadata provider is wired. trust-tiered (needs only hop PKs) and all the shape/tick presets work with no provider.