feat(adapters): 入向端点移交适配器 —— adapter.ingress + deserialize_request，内核主动让出

plan.md 的 R2 目标里有三行钩子契约，出向两条（serialize_response/serialize_chunk）
Phase 2 已落地，第三条 parse_request(protocol_body) -> unified 一直空着：
/v1/messages 的入向解析硬编码在内核 Go 里（anthropicRequest.toUnified），
Anthropic 改协议就得改内核。本次补齐这条对偶。

## 机制（与出向序列化对称）

- 适配器声明 adapter.ingress = "/v1/messages" 认领入站路径，并实现
  deserialize_request(raw_body) -> unified_json。
- 内核 handleMessages 经新增的 parseViaIngress 判定：
  - 有适配器认领该路径且实现了钩子 → 让出，解析由适配器完成；
  - 无适配器认领 → 内核内置解析运行（R5 兜底，未配置部署仍可服务）。
- 认领了但钩子执行失败 → 硬错误 400，不静默换用另一种协议解析
  （客户端说的正是该适配器声称支持的协议，悄悄换解析器只会错误调度）。
- 这是内核里唯一的新判定点，与 serializerFor 对称：出向选序列化器，入向选解析器。

## 内核改动

- internal/lua/vm.go：staticInfo 增 ingress 字段；inspectScript 读取；
  新增 VM.AdapterForIngress(path)（确定性选择：同名多认领取字典序最小）。
- internal/gateway/messages.go：handleMessages 改为先读 body → parseViaIngress
  （让出）→ 未认领时回落内置 anthropicRequest.toUnified。
  内置解析与 anthropicRequest 类型保留——它们是兜底，不是死代码。

## 适配器改动

- internal/lua/adapters/anthropic.lua：声明 adapter.ingress = "/v1/messages"，
  新增 deserialize_request，把内核原 Go 解析逻辑逐字段移植（system 两种拼写、
  content 原样透传、缺省 thinking={enabled:false}、缺 messages 报错）。
  内置 anthropic 默认随包播种，因此真实部署里内核总是让出给它，
  Go 侧 anthropicRequest.toUnified 退化为纯兜底。

## 测试

- gateway：TestMessagesIngressYieldsToBundledAnthropic（让出后仍 200）、
  TestAdapterForIngressClaim（声明可被找到、未声明路径为空、钩子存在）、
  TestMessagesKernelFallbackWhenNoAdapterClaims（移除认领适配器后内核兜底仍 200）。
- lua：TestAnthropicDeserializeRequestShape 逐字段核对 Lua 与 Go 产出一致
  （system 拼接、content、max_tokens/stream/temperature/top_p/top_k/stop_sequences、
  缺省 thinking），TestAnthropicDeserializeRejectsMissingMessages。
- 全量回归：gateway/lua/core/config/types/provider/scheduler/e2e 全绿。
- 文档：docs/lua-adapters.md 补「入向让出钩子」章节 + plan.md 台账更新。

internal/lua/adapters/anthropic.lua

@@ -5,6 +5,14 @@ adapter.endpoint = "/v1/messages"
 adapter.headers = {
     ["anthropic-version"] = "2023-06-01",
 }
+-- INGRESS CLAIM (inbound dual of serialize_*): this adapter owns parsing of the
+-- client-facing POST /v1/messages request body. The kernel keeps a built-in
+-- Anthropic parser as its fallback (R5) — that is what an unconfigured
+-- deployment uses — but once this adapter is loaded the kernel YIELDS, and the
+-- deserialize_request hook below is the single place Anthropic's REQUEST shape
+-- is understood. Iterating on that shape therefore means editing this file,
+-- never the kernel.
+adapter.ingress = "/v1/messages"
 
 -- ============================================================
 -- helpers
@@ -132,6 +140,79 @@ local function append_user(msgs, blocks)
     end
 end
 
+-- ============================================================
+-- deserialize_request  (Anthropic → unified)   [INGRESS]
+-- ============================================================
+-- The inbound dual of serialize_response. The kernel hands over the raw client
+-- body and receives the unified request; from there scheduling is identical to
+-- /v1/chat/completions. Ported field-for-field from the kernel's former
+-- built-in anthropicRequest.toUnified — which remains as the kernel fallback
+-- for an unconfigured deployment.
+function adapter.deserialize_request(raw_body)
+    local ok, req = pcall(json.decode, raw_body)
+    if not ok or type(req) ~= "table" then
+        error("invalid json")
+    end
+
+    -- system accepts Anthropic's two spellings: a plain string, or an array of
+    -- { type = "text", text = ... } blocks. Only text blocks contribute.
+    local system = ""
+    local sys = req.system
+    if type(sys) == "string" then
+        system = sys
+    elseif type(sys) == "table" then
+        for _, b in ipairs(sys) do
+            if type(b) == "table" and b.type == "text" and type(b.text) == "string" then
+                system = system .. b.text
+            end
+        end
+    end
+
+    local msgs = {}
+    local in_msgs = req.messages
+    if type(in_msgs) ~= "table" or #in_msgs == 0 then
+        error("messages is required")
+    end
+    for i, m in ipairs(in_msgs) do
+        if type(m) ~= "table" or type(m.role) ~= "string" or m.role == "" then
+            error("messages[" .. (i - 1) .. "].role is required")
+        end
+        -- Content is carried VERBATIM: a string stays a string, a block array
+        -- stays an array. The unified format keeps multimodal/tool blocks
+        -- untouched because only an adapter knows which block types the upstream
+        -- accepts. An absent/null content becomes the empty string.
+        local content = m.content
+        if content == nil or content == "" then
+            content = ""
+        end
+        table.insert(msgs, { role = m.role, content = content })
+    end
+
+    -- Anthropic's default is thinking OFF; emit that explicitly so the unified
+    -- request carries the client's intent rather than leaving it absent.
+    local thinking = req.thinking
+    if thinking == nil then
+        thinking = { enabled = false }
+    end
+
+    local out = {
+        model          = req.model or "",
+        messages       = msgs,
+        system         = system,
+        max_tokens     = req.max_tokens or 0,
+        stream         = req.stream == true,
+        temperature    = req.temperature,
+        top_p          = req.top_p,
+        top_k          = req.top_k or 0,
+        stop_sequences = req.stop_sequences,
+        tools          = req.tools,
+        tool_choice    = req.tool_choice,
+        thinking       = thinking,
+        extra_body     = {},
+    }
+    return json.encode(out)
+end
+
 -- ============================================================
 -- transform_request  (OpenAI → Anthropic)
 -- ============================================================