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
+}

submission.go

@@ -13,6 +13,15 @@ import (
 	"git.anthrove.art/public/go-fa-api/internal/urls"
 )
 
+// CategorizedTags groups FA's prefixed system tags by category. Names are
+// stored without their prefix (e.g. "s_hybrid_species" → Species "hybrid_species").
+type CategorizedTags struct {
+	Species    []string
+	Characters []string
+	Artists    []string
+	Types      []string
+}
+
 // Submission is a fully resolved FA submission as seen on /view/{id}/.
 type Submission struct {
 	ID              SubmissionID
@@ -27,6 +36,10 @@ type Submission struct {
 	Description     string // raw HTML; sanitise before rendering to a browser
 	DescriptionText string // plaintext convenience
 	Tags            []string
+	// CategorizedTags groups FA's prefixed system tags by category.
+	// FA emits these as tag-block entries inside div.submission-tags with
+	// prefixes s_ (species), c_ (character), a_/u_ (artist), and t_ (type).
+	CategorizedTags CategorizedTags
 	FileURL         string // absolute CDN URL; pass to Download
 	ThumbURL        string
 	Width           int // 0 if unknown / non-image

submission_parser.go

@@ -156,6 +156,32 @@ func parseSubmission(id SubmissionID, doc *goquery.Document) (*Submission, error
 		}
 	})
 
+	// Prefixed system tags FA renders these as tag-block anchors with a
+	// data-tag-name attribute carrying a leading single-letter prefix:
+	//   s_ species, c_ character, a_/u_ artist, t_ type.
+	// They are paired with a sibling <span class="tag-invalid"> and have no
+	// /search/ href, so they are skipped by the keyword pass above.
+	doc.Find("div.submission-tags a.tag-block[data-tag-name]").Each(func(_ int, a *goquery.Selection) {
+		raw := strings.TrimSpace(trimAttr(a, "data-tag-name"))
+		if len(raw) < 3 || raw[1] != '_' {
+			return
+		}
+		name := raw[2:]
+		if name == "" {
+			return
+		}
+		switch raw[0] {
+		case 's':
+			s.CategorizedTags.Species = append(s.CategorizedTags.Species, name)
+		case 'c':
+			s.CategorizedTags.Characters = append(s.CategorizedTags.Characters, name)
+		case 'a', 'u':
+			s.CategorizedTags.Artists = append(s.CategorizedTags.Artists, name)
+		case 't':
+			s.CategorizedTags.Types = append(s.CategorizedTags.Types, name)
+		}
+	})
+
 	// File URL FA renders a "Download" button in #submission-options that
 	// links to the canonical file for *every* submission type. For visual
 	// art it equals the #submissionImg source; for stories and music it's

submission_parser_test.go

@@ -54,6 +54,10 @@ const syntheticSubmissionHTML = `<html><body>
   <div>
     <span class="tags"><span><a href="javascript:void(0);" class="tag-block"></a><a href="/search/@keywords wolf">wolf</a></span></span>
     <span class="tags"><span><a href="javascript:void(0);" class="tag-block"></a><a href="/search/@keywords art">art</a></span></span>
+    <span class="tags"><span><a href="javascript:void(0);" data-tag-name="s_wolf" class="tag-block"></a><span class="tag-invalid">s_wolf</span></span></span>
+    <span class="tags"><span><a href="javascript:void(0);" data-tag-name="c_artwork_digital" class="tag-block"></a><span class="tag-invalid">c_artwork_digital</span></span></span>
+    <span class="tags"><span><a href="javascript:void(0);" data-tag-name="t_general_furry_art" class="tag-block"></a><span class="tag-invalid">t_general_furry_art</span></span></span>
+    <span class="tags"><span><a href="javascript:void(0);" data-tag-name="u_somefurry" class="tag-block"></a><span class="tag-invalid">u_somefurry</span></span></span>
   </div>
 </div>
 
@@ -110,6 +114,34 @@ func TestParseSubmission_Synthetic(t *testing.T) {
 	if !strings.Contains(sub.Description, "world") {
 		t.Errorf("Description missing expected content: %q", sub.Description)
 	}
+
+	catChecks := []struct {
+		name string
+		got  []string
+		want []string
+	}{
+		{"Species", sub.CategorizedTags.Species, []string{"wolf"}},
+		{"Characters", sub.CategorizedTags.Characters, []string{"artwork_digital"}},
+		{"Types", sub.CategorizedTags.Types, []string{"general_furry_art"}},
+		{"Artists", sub.CategorizedTags.Artists, []string{"somefurry"}},
+	}
+	for _, c := range catChecks {
+		if !equalStrings(c.got, c.want) {
+			t.Errorf("CategorizedTags.%s = %v; want %v", c.name, c.got, c.want)
+		}
+	}
+}
+
+func equalStrings(a, b []string) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	for i := range a {
+		if a[i] != b[i] {
+			return false
+		}
+	}
+	return true
 }
 
 // TestParseSubmission_FavoritedState verifies parseSubmission reports the