Add Anthropic platform: wire-served metadata via listing dialect (provider 2)

The 5th registry row: id "anthropic", ANTHROPIC_API_KEY > auth.json
"anthropic" scope, api.anthropic.com/v1 with KIGI_ANTHROPIC_BASE_URL
override, Messages dialect. Two new spec dimensions most future rows
reuse: ListingDialect (Anthropic's /v1/models wants x-api-key +
anthropic-version headers, ?limit=1000, and its own response shape) and
PlatformKeyHeader (Bearer vs x-api-key across listing/validation/
inference, with auth_scheme stamped onto entries).

The 2026 Anthropic listing serves real metadata: the adapter maps
max_input_tokens, per-level effort capabilities (low..max as the menu,
xhigh/max distinct), thinking/image flags — and enrichment fills only
genuine wire gaps (e2e pins wire-1M beating enrichment, and a zero
context filled to 200k).

Two review-confirmed defects fixed red-green:
- Output caps were dropped at three layers, so every sub-128K-output
  model (64k Haiku, legacy models) would 400 on EVERY request against
  the sampler's 128K max_tokens default. Wire max_tokens and enrichment
  limit.output now flow to entry.max_completion_tokens.
- An explicit wire effort-decline was indistinguishable from wire
  silence, letting enrichment inject effort menus pre-4.6 models reject
  (adaptive thinking 400). The adapter now emits a decline sentinel
  (support:false) that enrichment respects — proven end to end.

Also: the Messages client now sends anthropic-version (previously never
sent — real api.anthropic.com rejects such requests; pinned across all
three scheme/backend quadrants), key validation builds per-key-header
requests, missing listing data fails fast, empty-id ghosts drop with a
warning, kimi-import recognizes api.anthropic.com as built-in
automatically.

crates/codegen/kigi-sampling-types/src/types.rs

@@ -934,6 +934,10 @@ pub fn normalize_effort_echo(value: &mut Value) {
     }
 }
 
+/// The `anthropic-version` header value kigi speaks on Anthropic-style
+/// wires (Messages inference and the /v1/models listing).
+pub const ANTHROPIC_VERSION: &str = "2023-06-01";
+
 pub const REASONING_EFFORT_META_KEY: &str = "reasoningEffort";
 pub const SUPPORTS_REASONING_EFFORT_META_KEY: &str = "supportsReasoningEffort";