Auth level 1: registry auth spec and a single apply_auth

[cunninghamcard-bit]

Part of #166 (track B); RFC-0033 §8.6 level 1; extends RFC-0032 §3.3.

The registry can only express one kind of auth: env_var → read an API key → Authorization: Bearer. Everything else is hand-written per provider with no shared function: azure sends api-key, google x-goog-api-key, lmnt / exa_ai / parallel_ai / you_com / tinyfish x-api-key, bedrock-family providers each call bedrock/sigv4.rs, vertex takes an externally minted access token, and the 33 local-service wrappers send a PLACEHOLDER_API_KEY. Twelve header-building sites, zero reuse.

Work

1. Row schema: replace env_var with

   "auth": { "kind": "api_key", "env": "AZURE_API_KEY", "header": "api-key" }

   kind ∈ api_key (header defaults to Bearer; may be x-api-key, api-key, x-goog-api-key), none (local services; retire every PLACEHOLDER_API_KEY), sigv4 (params.region), bearer_token (externally supplied token, vertex). Legacy env_var keeps loading as api_key + Bearer during the transition.
2. aimux_provider_utils::apply_auth(headers: &mut HeaderMap, spec: &AuthSpec, credential: &Credential) — the single place that turns a credential into headers (SigV4 signing stays in bedrock/sigv4.rs but is invoked from here). Replace the twelve hand-written sites.
3. Registry validation test: every row's auth is well-formed; none rows never send an Authorization header (unit test on apply_auth).

Lands with or right after B1 (protocol column). Net deletion expected.