feat(uadetect): add FromRequest for full agent detection

The user agent identifies itself through multiple signals — the
User-Agent header and query parameters (?os, ?arch). FromRequest
unifies both, with explicit query params taking precedence.

internal/uadetect/uadetect.go

@@ -1,15 +1,19 @@
-// Package uadetect identifies the requesting system's OS, CPU architecture,
-// and libc from its User-Agent string.
+// Package uadetect identifies the requesting agent's OS, CPU architecture,
+// and libc so the server can select the correct release artifact.
 //
-// Webi's bootstrap scripts send "$(uname -srm)" as the User-Agent, e.g.
-// "Darwin 23.1.0 arm64" or "Linux 6.1.0 x86_64". This package parses those
-// into [buildmeta.OS], [buildmeta.Arch], and [buildmeta.Libc] values so the
-// server can select the correct release artifact.
+// An agent identifies itself through multiple signals:
+//   - The User-Agent header: Webi's bootstrap scripts send "$(uname -srm)",
+//     e.g. "Darwin 23.1.0 arm64". Browsers, curl, and PowerShell send their
+//     own UA strings.
+//   - Query parameters: ?os=linux&arch=arm64 are an explicit declaration
+//     that takes precedence over the header.
 //
-// Also handles non-uname agents like "PowerShell/7.3.0" and "MS AMD64".
+// Use [FromRequest] to detect from an HTTP request (preferred).
+// Use [Parse] to detect from a raw UA string.
 package uadetect
 
 import (
+	"net/http"
 	"strings"
 
 	"github.com/webinstall/webi-installers/internal/buildmeta"
@@ -22,6 +26,27 @@ type Result struct {
 	Libc buildmeta.Libc
 }
 
+// FromRequest detects the agent's platform from an HTTP request.
+// Query parameters ?os and ?arch override the User-Agent header.
+func FromRequest(r *http.Request) Result {
+	qOS := r.URL.Query().Get("os")
+	qArch := r.URL.Query().Get("arch")
+
+	var ua string
+	switch {
+	case qOS != "" && qArch != "":
+		ua = qOS + " " + qArch
+	case qOS != "":
+		ua = qOS
+	case qArch != "":
+		ua = qArch
+	default:
+		ua = r.Header.Get("User-Agent")
+	}
+
+	return Parse(ua)
+}
+
 // Parse extracts OS, arch, and libc from a User-Agent string.
 func Parse(ua string) Result {
 	if ua == "-" {

internal/uadetect/uadetect_test.go

@@ -1,6 +1,7 @@
 package uadetect_test
 
 import (
+	"net/http"
 	"testing"
 
 	"github.com/webinstall/webi-installers/internal/buildmeta"
@@ -103,6 +104,58 @@ func TestLibc(t *testing.T) {
 	}
 }
 
+func TestFromRequest(t *testing.T) {
+	tests := []struct {
+		name   string
+		ua     string // User-Agent header
+		query  string // raw query string
+		wantOS buildmeta.OS
+		wantAr buildmeta.Arch
+	}{
+		{
+			name:   "UA header only",
+			ua:     "Darwin 23.1.0 arm64",
+			wantOS: buildmeta.OSDarwin,
+			wantAr: buildmeta.ArchARM64,
+		},
+		{
+			name:   "query params override UA",
+			ua:     "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7)",
+			query:  "os=linux&arch=aarch64",
+			wantOS: buildmeta.OSLinux,
+			wantAr: buildmeta.ArchARM64,
+		},
+		{
+			name:   "os param only",
+			ua:     "curl/8.1.2",
+			query:  "os=windows",
+			wantOS: buildmeta.OSWindows,
+		},
+		{
+			name:   "arch param only",
+			ua:     "curl/8.1.2",
+			query:  "arch=arm64",
+			wantAr: buildmeta.ArchARM64,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			req, _ := http.NewRequest("GET", "http://example.com/api?"+tt.query, nil)
+			if tt.ua != "" {
+				req.Header.Set("User-Agent", tt.ua)
+			}
+			got := uadetect.FromRequest(req)
+			if tt.wantOS != "" && got.OS != tt.wantOS {
+				t.Errorf("OS = %q, want %q", got.OS, tt.wantOS)
+			}
+			if tt.wantAr != "" && got.Arch != tt.wantAr {
+				t.Errorf("Arch = %q, want %q", got.Arch, tt.wantAr)
+			}
+		})
+	}
+}
+
 func TestFullParse(t *testing.T) {
 	r := uadetect.Parse("Darwin 23.1.0 arm64")
 	if r.OS != buildmeta.OSDarwin {