feat(submission): parse FA's prefixed system tags into CategorizedTags

FA renders its species/character/artist/type system tags as tag-block
anchors with a data-tag-name carrying a single-letter prefix
(s_/c_/a_-u_/t_) and a sibling tag-invalid span instead of a /search/
link. The existing keyword pass skips them, so they were lost.

Adds a Submission.CategorizedTags field exposing the four buckets with
the prefix stripped, plus an examples/categorized_tags runnable demo.

examples/categorized_tags/main.go

@@ -0,0 +1,90 @@
+// categorized_tags demonstrates how the SDK groups FA's prefixed system
+// tags into the four CategorizedTags buckets: Species (s_), Characters (c_),
+// Artists (a_/u_), and Types (t_). Each bucket is printed on its own so the
+// example covers every aspect of the feature.
+//
+// Runs anonymously by default. Set FA_A / FA_B (and ideally CF_CLEARANCE +
+// FA_UA) to authenticate when the target submission requires it.
+//
+//	go run ./examples/categorized_tags 12345678
+package main
+
+import (
+	"context"
+	"fmt"
+	"log"
+	"os"
+	"strconv"
+	"strings"
+
+	fa "git.anthrove.art/public/go-fa-api"
+)
+
+func main() {
+	if len(os.Args) < 2 {
+		log.Fatalf("usage: %s <submission-id>", os.Args[0])
+	}
+	id, err := strconv.ParseInt(os.Args[1], 10, 64)
+	if err != nil {
+		log.Fatalf("invalid submission id: %v", err)
+	}
+
+	opts := []fa.Option{fa.WithUserAgent("go-fa-api-example/0.1")}
+	if a, b := os.Getenv("FA_A"), os.Getenv("FA_B"); a != "" && b != "" {
+		log.Printf("using FA_A/FA_B cookies for authenticated request")
+		opts = []fa.Option{
+			fa.WithCookies(fa.Cookies{A: a, B: b}),
+			fa.WithCloudflare(fa.CFCookies{Clearance: os.Getenv("CF_CLEARANCE")}),
+			fa.WithUserAgent(envOr("FA_UA", "go-fa-api-example/0.1")),
+		}
+	}
+	client := fa.New(opts...)
+
+	sub, err := client.GetSubmission(context.Background(), fa.SubmissionID(id))
+	if err != nil {
+		log.Fatalf("GetSubmission: %v", err)
+	}
+
+	fmt.Printf("%s\nby %s\n\n", sub.Title, sub.Author.DisplayName)
+
+	fmt.Println("=== Plain keyword tags (no prefix) ===")
+	if len(sub.Tags) == 0 {
+		fmt.Println("  (none)")
+	} else {
+		fmt.Println("  " + strings.Join(sub.Tags, ", "))
+	}
+
+	ct := sub.CategorizedTags
+	fmt.Println()
+	fmt.Println("=== Species (s_) ===")
+	printBucket(ct.Species, "s_")
+
+	fmt.Println()
+	fmt.Println("=== Characters (c_) ===")
+	printBucket(ct.Characters, "c_")
+
+	fmt.Println()
+	fmt.Println("=== Artists (a_ / u_) ===")
+	printBucket(ct.Artists, "a_")
+
+	fmt.Println()
+	fmt.Println("=== Types (t_) ===")
+	printBucket(ct.Types, "t_")
+}
+
+func printBucket(items []string, prefix string) {
+	if len(items) == 0 {
+		fmt.Println("  (none)")
+		return
+	}
+	for _, v := range items {
+		fmt.Printf("  %s%s\n", prefix, v)
+	}
+}
+
+func envOr(key, fallback string) string {
+	if v := os.Getenv(key); v != "" {
+		return v
+	}
+	return fallback
+}