feat: initial extract of shared anthropic-compat client + translator

Extracted from Plexum-minimax-provider and Plexum-kimi-provider (both
had near-identical copies under internal/anthropic + internal/translate).
Future provider plugins (Qwen, Doubao, …) import these two packages
instead of re-implementing the protocol.

anthropic/ (~220 LOC + 6 tests):
- minimal HTTP+SSE Anthropic Messages client (POST /v1/messages,
  stream:true, parses event:/data: SSE)
- Client.UserAgent + Client.ExtraHeaders fields for per-backend
  customization (Kimi needs "claude-code/0.1.0"; MiniMax is flexible)
- non-2xx surfaces as Go error; mid-stream errors as final
  Event{Type:"error"}

translate/ (~220 LOC):
- CanonicalToAnthropic: TurnRequest → MessagesRequest
- blockToAnthropic: TextBlock / ToolUseBlock / ToolResultBlock /
  ThinkingBlock → loose ContentBlock map; preserves signatures
- Translator: per-turn state machine; anthropic.Event stream →
  canonical.TurnEvent stream (text + thinking + tool_use deltas;
  signature_delta capture; message_delta stop_reason + usage)

Both MiniMax and Kimi now import from here; their internal/* dirs are
gone. Live verified after refactor — both still answer "ready" via
plexum say.

README.md

@@ -0,0 +1,72 @@
+# Plexum-anthropic-compat-client
+
+Shared HTTP+SSE Anthropic Messages API client + canonical ↔ Anthropic
+translator. Used by Plexum provider plugins that talk to any
+"Anthropic-compatible" endpoint (the one Anthropic itself, MiniMax,
+Kimi, Qwen, etc. all expose at `/v1/messages`).
+
+Each provider plugin imports these two packages and adds:
+- its own `cmd/plexum-<name>-provider-plugin/main.go` entry
+- a list of supported model ids in the manifest
+- an endpoint URL + auth key + optional User-Agent override
+
+## Packages
+
+### `anthropic/`
+
+Minimal HTTP+SSE client. Single entry point:
+
+```go
+client := anthropic.New(baseURL, apiKey)
+client.UserAgent = "claude-code/0.1.0" // optional
+events, err := client.StreamMessages(ctx, anthropic.MessagesRequest{
+    Model: "MiniMax-M2.7", MaxTokens: 4096,
+    Messages: []anthropic.Message{...},
+})
+for ev := range events { /* anthropic.Event */ }
+```
+
+Features:
+- POST `/v1/messages` with `stream: true`
+- Parses standard SSE frames (`event:` + `data:` + blank line)
+- Surfaces non-2xx as HTTP error (before channel opens)
+- Surfaces mid-stream errors as `Event{Type:"error"}` final entry
+- `UserAgent` + `ExtraHeaders` fields for per-backend customization
+- 5min per-call timeout
+
+### `translate/`
+
+Bidirectional canonical-types ↔ Anthropic-types adapter:
+
+- `CanonicalToAnthropic(req canonical.TurnRequest, modelID, defaultMaxTokens) → anthropic.MessagesRequest`
+- `Translator` state machine consumes `<-chan anthropic.Event`,
+  emits `[]canonical.TurnEvent` per event (handles text / thinking /
+  tool_use / signature deltas + per-block close)
+
+## Usage
+
+In a provider plugin's `go.mod`:
+
+```
+require git.hangman-lab.top/hzhang/Plexum-anthropic-compat-client v0.0.0
+replace git.hangman-lab.top/hzhang/Plexum-anthropic-compat-client => ../Plexum-anthropic-compat-client
+```
+
+In code:
+
+```go
+import (
+    "git.hangman-lab.top/hzhang/Plexum-anthropic-compat-client/anthropic"
+    "git.hangman-lab.top/hzhang/Plexum-anthropic-compat-client/translate"
+)
+```
+
+## History
+
+Extracted from `Plexum-minimax-provider` and `Plexum-kimi-provider` —
+both had a near-identical copy. New providers (Qwen, Doubao, …) drop in
+without re-implementing the protocol.
+
+## License
+
+Same as Plexum.