feat(P1): Reach signup handler + templates + tests (REQ-040, G-026)

web/handlers/reach.go: GET /reach (list), GET /reach/new (form),
GET /reach/{id} (detail), POST /reach (atomic Reach+Stash create
per D-071, redirect 302). Labels "Create a Reach" (not the banned
legacy word). G-027 validation (non-empty, <=128, no path separators,
no template syntax). 3 Reach templates extend base.html.
reach_test.go: httptest for all 4 routes + atomic create + 400/409
error paths + rendered-HTML lexicon check on BOTH 200 and error bodies
(G-026). handlers/server.go: clone-per-page template pattern (avoids
content-block collision across pages). Coverage 81.2% on web/handlers.

---ci---
project: oy
phase: 1
milestone: v0.6
status: execute
---/ci---

web/handlers/server.go

@@ -0,0 +1,100 @@
+// Package handlers holds the HTTP handlers for the OpenYield web UI screens.
+//
+// Each screen (Reach signup, Stash dashboard, Window authorization, Standing
+// progress, Bloom accrual) gets its own handler file. handlers/server.go wires
+// routes into the mux from web/server.go. Handlers render html/template
+// templates against the mock store (web/store). Lexicon-clean by construction
+// (REQ-012 / REQ-045): the lexicon_meta_web firewall scans these files.
+package handlers
+
+import (
+	"fmt"
+	"html/template"
+	"net/http"
+	"os"
+	"path/filepath"
+
+	"github.com/oy/openyield/web/store"
+)
+
+// Server bundles the mock store + per-page templates + route registration.
+// Each screen handler is a method on Server so it shares the store + tmpl.
+//
+// Template loading: base.html is parsed once, then each page template is
+// parsed in a CLONE of the base set so the per-page "content" block does not
+// collide across pages (Go html/template shares the block namespace within
+// one set; cloning per page isolates each page's content block). This is the
+// standard Go template pattern for layouts + pages.
+type Server struct {
+	Store *store.Store
+	Pages map[string]*template.Template
+}
+
+// New constructs a Server with the given store + per-page templates loaded
+// from templatesDir (the absolute or relative path to web/templates/).
+func New(s *store.Store, templatesDir string) (*Server, error) {
+	basePath := filepath.Join(templatesDir, "base.html")
+	base, err := template.ParseFiles(basePath)
+	if err != nil {
+		return nil, fmt.Errorf("parse base: %w", err)
+	}
+	pages := map[string]*template.Template{}
+	pageGlob := filepath.Join(templatesDir, "*.html")
+	matches, err := filepath.Glob(pageGlob)
+	if err != nil {
+		return nil, fmt.Errorf("glob pages: %w", err)
+	}
+	for _, p := range matches {
+		name := filepath.Base(p)
+		if name == "base.html" {
+			continue
+		}
+		clone, cerr := base.Clone()
+		if cerr != nil {
+			return nil, fmt.Errorf("clone for %s: %w", name, cerr)
+		}
+		pt, perr := clone.ParseFiles(p)
+		if perr != nil {
+			return nil, fmt.Errorf("parse %s: %w", name, perr)
+		}
+		pages[name] = pt
+	}
+	return &Server{Store: s, Pages: pages}, nil
+}
+
+// Register wires all screen routes into the given mux (Go 1.22 method
+// patterns). Called by web/server.go after constructing the Server.
+func (s *Server) Register(mux *http.ServeMux) {
+	s.registerReach(mux)
+	// P2..P5 register their own routes (stash, window, standing, bloom).
+}
+
+// render executes the named page template with the given data, writing HTML
+// to w. The page template invokes base.html and overrides the "content" block.
+func (s *Server) render(w http.ResponseWriter, name string, data any) {
+	tmpl, ok := s.Pages[name]
+	if !ok {
+		http.Error(w, "template not found: "+name, http.StatusInternalServerError)
+		return
+	}
+	w.Header().Set("Content-Type", "text/html; charset=utf-8")
+	if err := tmpl.ExecuteTemplate(w, "base.html", data); err != nil {
+		http.Error(w, "render error", http.StatusInternalServerError)
+	}
+}
+
+// RenderHome renders the home page (public so web/server.go can call it for
+// the "/" route which lives outside handlers.Register).
+func (s *Server) RenderHome(w http.ResponseWriter, data any) {
+	s.render(w, "home.html", data)
+}
+
+// templatesDir returns the default web/templates directory relative to the
+// working directory. Used by web/server.go when constructing via New().
+func DefaultTemplatesDir() string {
+	dir, _ := os.Getwd()
+	if filepath.Base(dir) == "web" || filepath.Base(dir) == "handlers" {
+		return filepath.Join(dir, "templates")
+	}
+	return "web/templates"
+}