feat: search — GET /search.json with grouped hits

Client.Search(term, page) url-encodes the term (operators like
@user / #category / order:latest pass through) and decodes the four
hit groups Discourse returns: posts (with blurb and highlighted
topic title), topics, users and categories. Empty term is rejected
locally with ErrInvalidRequest; server failures keep the typed
error classes. Fake-server tests cover encoding, parsing and the
403 path.

Part of Redmine 507 (Discourse Go client): search is how the 495
briefing pipeline finds landing spots and prior threads.

search.go

@@ -0,0 +1,74 @@
+// Search: GET /search.json — full-text search across posts, topics,
+// users and categories. The term supports Discourse's search operators
+// (@user, #category, order:latest, ...); they pass through untouched.
+package discourse
+
+import (
+	"context"
+	"fmt"
+	"net/url"
+	"strconv"
+	"strings"
+)
+
+// SearchRequest is the GET /search.json query. Term is required; Page is
+// 1-based (0 means "leave unset" — Discourse then returns page 1).
+type SearchRequest struct {
+	Term string
+	Page int
+}
+
+// SearchPost is one post hit: the match lives in Blurb (a plain-text
+// excerpt around the match) while TopicTitleHeadline carries the topic
+// title with <em> highlights around matched words.
+type SearchPost struct {
+	ID                 int    `json:"id"`
+	TopicID            int    `json:"topic_id"`
+	PostNumber         int    `json:"post_number"`
+	Username           string `json:"username"`
+	Name               string `json:"name"`
+	Blurb              string `json:"blurb"`
+	TopicTitleHeadline string `json:"topic_title_headline"`
+	CreatedAt          string `json:"created_at"`
+	LikeCount          int    `json:"like_count"`
+}
+
+// SearchTopic is one topic hit.
+type SearchTopic struct {
+	ID    int    `json:"id"`
+	Title string `json:"title"`
+	Slug  string `json:"slug"`
+}
+
+// SearchUser is one user hit.
+type SearchUser struct {
+	ID       int    `json:"id"`
+	Username string `json:"username"`
+	Name     string `json:"name"`
+}
+
+// SearchResult groups the four hit lists Discourse returns; whichever
+// classes matched are non-empty, the rest are nil.
+type SearchResult struct {
+	Posts      []SearchPost  `json:"posts"`
+	Topics     []SearchTopic `json:"topics"`
+	Users      []SearchUser  `json:"users"`
+	Categories []Category    `json:"categories"`
+}
+
+// Search runs a full-text search and returns the grouped hits.
+func (c *Client) Search(ctx context.Context, req SearchRequest) (*SearchResult, error) {
+	if strings.TrimSpace(req.Term) == "" {
+		return nil, fmt.Errorf("%w: search term is required", ErrInvalidRequest)
+	}
+	q := url.Values{}
+	q.Set("term", req.Term)
+	if req.Page > 1 {
+		q.Set("page", strconv.Itoa(req.Page))
+	}
+	var res SearchResult
+	if err := c.Get(ctx, "/search.json?"+q.Encode(), &res); err != nil {
+		return nil, err
+	}
+	return &res, nil
+}