feat(P1): web foundation — main.go, server.go, HTMX vendored, base+home templates (REQ-040)

Go 1.22 net/http.ServeMux + html/template + http.FileServer for static.
HTMX 2.0.10 vendored as web/static/htmx.min.js (NOT go get; G-006).
base.html layout with nav to all 5 screens. home.html overview.

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

web/static/style.css

@@ -0,0 +1,78 @@
+/* style.css — OpenYield web UI minimal styling (lexicon-clean).
+   No banned terms in comments or class names (REQ-012/REQ-045). */
+
+:root {
+  --bg: #0d1117;
+  --panel: #161b22;
+  --ink: #c9d1d9;
+  --muted: #8b949e;
+  --accent: #58a6ff;
+  --line: #30363d;
+}
+
+* { box-sizing: border-box; }
+
+body {
+  margin: 0;
+  font-family: system-ui, -apple-system, sans-serif;
+  background: var(--bg);
+  color: var(--ink);
+  line-height: 1.5;
+}
+
+a { color: var(--accent); text-decoration: none; }
+a:hover { text-decoration: underline; }
+
+header.nav {
+  border-bottom: 1px solid var(--line);
+  padding: 0.75rem 1.5rem;
+  display: flex;
+  gap: 1.25rem;
+  align-items: center;
+  background: var(--panel);
+}
+header.nav .brand { font-weight: 600; color: var(--ink); }
+header.nav a { color: var(--muted); }
+header.nav a:hover { color: var(--accent); }
+
+main { max-width: 960px; margin: 2rem auto; padding: 0 1.5rem; }
+
+footer {
+  border-top: 1px solid var(--line);
+  padding: 1rem 1.5rem;
+  color: var(--muted);
+  font-size: 0.85rem;
+  text-align: center;
+}
+
+.panel {
+  background: var(--panel);
+  border: 1px solid var(--line);
+  border-radius: 6px;
+  padding: 1.25rem;
+  margin-bottom: 1.5rem;
+}
+
+table { width: 100%; border-collapse: collapse; }
+th, td { text-align: left; padding: 0.5rem 0.75rem; border-bottom: 1px solid var(--line); }
+th { color: var(--muted); font-weight: 600; font-size: 0.85rem; text-transform: uppercase; letter-spacing: 0.04em; }
+
+form .field { margin-bottom: 1rem; }
+form label { display: block; margin-bottom: 0.25rem; color: var(--muted); font-size: 0.9rem; }
+form input[type=text], form input[type=password] {
+  width: 100%; max-width: 32rem;
+  padding: 0.5rem 0.65rem;
+  background: var(--bg);
+  border: 1px solid var(--line);
+  border-radius: 4px;
+  color: var(--ink);
+  font-family: monospace;
+}
+button, .btn {
+  background: var(--accent); color: #0d1117; border: none;
+  padding: 0.5rem 1rem; border-radius: 4px; font-weight: 600; cursor: pointer;
+}
+button:hover, .btn:hover { opacity: 0.9; text-decoration: none; }
+
+.error { color: #f85149; }
+.muted { color: var(--muted); }