perf(client): static headers + bool fast-path in decodeResult

Two changes on the Call hot path:

* Replace httpReq.Header.Set("Content-Type", "application/json") (and Accept) with direct map writes against a package-level []string. Both keys are already canonical so the canonicalising path inside Header.Set was pure overhead; saves the per-call []string{val} allocation x2.

* Add a bool fast-path in decodeResult: ~60% of Telegram methods return bool, and the API emits the envelope with no whitespace, so a bytes.Equal check against the two canonical bodies short-circuits the generic Result[bool] Unmarshal entirely. any(true).(Resp) does not box thanks to Go's static bool interface values.

Combined effect on Call_BoolResponse: 18 -> 14 allocs/op, 634ns -> 526ns. DecodeResult_Bool isolation bench: 50ns / 2 allocs -> 2.87ns / 0 allocs.

.benchstats/after_step1.txt

@@ -0,0 +1,17 @@
+After: static-header-slices + bool-fast-path
+goos: darwin
+goarch: arm64
+pkg: github.com/lukaszraczylo/go-telegram/client
+cpu: Apple M4 Max
+BenchmarkCall_BoolResponse-16      	 4597374	       526.3 ns/op	    1842 B/op	      14 allocs/op
+BenchmarkCall_StructResponse-16    	 3740096	       679.6 ns/op	    1973 B/op	      16 allocs/op
+BenchmarkEncodeJSONBody-16         	41997352	        58.35 ns/op	      96 B/op	       2 allocs/op
+BenchmarkDecodeResult_Bool-16      	863273641	         2.872 ns/op	       0 B/op	       0 allocs/op
+BenchmarkDecodeResult_Struct-16    	19988096	       100.3 ns/op	     144 B/op	       2 allocs/op
+
+Deltas vs baseline (allocs/op):
+  Call_BoolResponse:    18 -> 14 (-4)
+  Call_StructResponse:  18 -> 16 (-2)
+  DecodeResult_Bool:     2 -> 0  (-2, also -94% ns)
+  DecodeResult_Struct:   2 -> 2  (flat)
+  EncodeJSONBody:        2 -> 2  (flat)

client/call.go

@@ -10,6 +10,12 @@ import (
 	"reflect"
 )
 
+var (
+	headerJSONValue = []string{"application/json"}
+	rawOKTrueBody   = []byte(`{"ok":true,"result":true}`)
+	rawOKFalseBody  = []byte(`{"ok":true,"result":false}`)
+)
+
 // Call is the single point through which every Telegram Bot API method
 // invocation flows. It marshals the request, signs the URL with the bot
 // token, dispatches via HTTPDoer, decodes the Result envelope, and
@@ -44,8 +50,8 @@ func Call[Req any, Resp any](ctx context.Context, b *Bot, method string, req Req
 	if err != nil {
 		return zero, &NetworkError{Err: err}
 	}
-	httpReq.Header.Set("Content-Type", "application/json")
-	httpReq.Header.Set("Accept", "application/json")
+	httpReq.Header["Content-Type"] = headerJSONValue
+	httpReq.Header["Accept"] = headerJSONValue
 
 	resp, err := b.http.Do(httpReq)
 	if err != nil {
@@ -91,8 +97,8 @@ func CallRaw[Req any](ctx context.Context, b *Bot, method string, req Req) (json
 	if err != nil {
 		return nil, &NetworkError{Err: err}
 	}
-	httpReq.Header.Set("Content-Type", "application/json")
-	httpReq.Header.Set("Accept", "application/json")
+	httpReq.Header["Content-Type"] = headerJSONValue
+	httpReq.Header["Accept"] = headerJSONValue
 
 	resp, err := b.http.Do(httpReq)
 	if err != nil {
@@ -139,8 +145,22 @@ func encodeJSONBody(codec Codec, req any) (io.Reader, error) {
 
 // decodeResult unmarshals raw into Result[Resp] and translates non-OK
 // responses into *APIError.
+//
+// Bool fast path: ~60% of Telegram methods return bool. The Telegram API
+// emits the result envelope with no whitespace, so a byte-equality check
+// against the two canonical bodies skips the generic Unmarshal entirely.
+// Anything that doesn't match exactly (e.g. responses with extra fields,
+// errors) falls through to the slow path.
 func decodeResult[Resp any](codec Codec, raw []byte) (Resp, error) {
 	var zero Resp
+	if _, isBool := any(zero).(bool); isBool {
+		switch {
+		case bytes.Equal(raw, rawOKTrueBody):
+			return any(true).(Resp), nil
+		case bytes.Equal(raw, rawOKFalseBody):
+			return any(false).(Resp), nil
+		}
+	}
 	var env Result[Resp]
 	if err := codec.Unmarshal(raw, &env); err != nil {
 		return zero, &ParseError{Err: err, Body: copyBody(raw)}

client/multipart.go

@@ -69,7 +69,7 @@ func callMultipart[Resp any](ctx context.Context, b *Bot, method string, mp mult
 		return zero, &NetworkError{Err: err}
 	}
 	req.Header.Set("Content-Type", mw.FormDataContentType())
-	req.Header.Set("Accept", "application/json")
+	req.Header["Accept"] = headerJSONValue
 
 	resp, err := b.http.Do(req)
 	if err != nil {
@@ -125,7 +125,7 @@ func callMultipartRaw(ctx context.Context, b *Bot, method string, mp multipartRe
 		return nil, &NetworkError{Err: err}
 	}
 	req.Header.Set("Content-Type", mw.FormDataContentType())
-	req.Header.Set("Accept", "application/json")
+	req.Header["Accept"] = headerJSONValue
 
 	resp, err := b.http.Do(req)
 	if err != nil {