docs(auth): document Express cookie package

auth/expresscookie/LICENSE

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+expresscookie
+
+Authored in 2026 by AJ ONeal.
+
+expresscookie is made available under the terms of any of the following
+licenses, at your option:
+
+1. CC0 1.0 Universal
+   https://creativecommons.org/publicdomain/zero/1.0/
+
+2. MIT License
+   https://opensource.org/license/mit/
+
+3. Apache License 2.0
+   https://www.apache.org/licenses/LICENSE-2.0
+
+SPDX-License-Identifier: CC0-1.0 OR MIT OR Apache-2.0

auth/expresscookie/README.md

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+# expresscookie
+
+A general-purpose package for signing and verifying secure cookies in all web applications. Its format is also compatible with the Node.js npm package [`cookie-signature`](https://www.npmjs.com/package/cookie-signature), as used by Express session middleware.
+
+Standard HMAC-SHA256 signature:
+
+```
+secret:       example-secret-16
+string:       Hello, World!
+payload:      SGVsbG8sIFdvcmxkIQ==
+signed value: s:SGVsbG8sIFdvcmxkIQ==.F/bW1t2GXhUIqykISYbB+FMA3lLquegPU4jYjVLsnXs
+cookie value: s%3ASGVsbG8sIFdvcmxkIQ%3D%3D.F%2FbW1t2GXhUIqykISYbB%2BFMA3lLquegPU4jYjVLsnXs
+```
+
+## Usage
+
+```sh
+go get github.com/therootcompany/golib/auth/expresscookie@latest
+```
+
+Initialize the secret once during application setup:
+
+```go
+secret, err := expresscookie.NewSecret([]byte(os.Getenv("COOKIE_SECRET")))
+```
+
+> **Payload:** Encode payloads before signing (e.g., base64, JSON). The
+> package signs raw bytes as supplied, but the behavior of non-encoded plain
+> strings is undefined — in particular, raw payloads containing characters
+> such as spaces may not interoperate with Node.js, which uses
+> `encodeURIComponent` rather than query-string escaping. With zero-valued
+> options, the cookie has zero-valued HTTP attributes except for the security
+> defaults applied by `Sign`. These convenience defaults may change.
+
+```go
+http.HandleFunc("GET /", func(w http.ResponseWriter, r *http.Request) {
+	cookie, err := r.Cookie("session")
+	if err != nil {
+		return
+	}
+	signed, err := expresscookie.Parse(cookie.Value)
+	if err != nil {
+		return
+	}
+	payload, err := signed.Verify(secret)
+	if err != nil {
+		http.Error(w, "invalid session", http.StatusUnauthorized)
+		return
+	}
+	_, _ = w.Write(payload)
+})
+```
+
+Sign and send a cookie:
+
+```go
+payload := base64.StdEncoding.AppendEncode(nil, []byte(`{"user":"123"}`))
+cookie, err := expresscookie.New("session", payload, http.Cookie{
+	Expires: time.Now().Add(time.Hour),
+})
+if err != nil {
+	return
+}
+signed := cookie.Sign(secret)
+http.SetCookie(w, &signed.Cookie)
+```
+
+`DecodeHexSecret` decodes and validates an `APP_SECRET`-style hexadecimal key.
+
+## License
+
+Authored in 2026 by AJ ONeal.
+
+expresscookie is made available under the terms of any of the following
+licenses, at your option:
+
+1. [CC0 1.0 Universal](https://creativecommons.org/publicdomain/zero/1.0/)
+2. [MIT License](https://opensource.org/license/mit/)
+3. [Apache License 2.0](https://www.apache.org/licenses/LICENSE-2.0)
+
+SPDX-License-Identifier: CC0-1.0 OR MIT OR Apache-2.0

auth/expresscookie/doc.go

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+// Package expresscookie is a general-purpose secure-cookie package for web
+// applications. Payload bytes are signed as supplied, then the signed value is
+// URL-escaped for cookie transport. The format is compatible with the Node.js
+// npm cookie-signature package used by Express session middleware.
+//
+// Payloads should be encoded by the caller (e.g., base64, JSON) before signing.
+// The behavior of non-encoded plain strings is undefined; in particular, raw
+// payloads containing characters such as spaces may not interoperate with
+// Node.js, which uses encodeURIComponent rather than query-string escaping.
+package expresscookie