addrkit: strict ordinal matching, LLM-variant guard, externalized prompt

- match: require candidate ordinal to equal the query ordinal in fuzzy search
  (deterministic typo recall 73% -> 80%, no wrong houses)
- resolver: compatibleVariant guard — LLM may only fix street spelling, keeping
  the same words, ordinal and house unit; damerau distance for transpositions
- llm: prompt moved out of code (go:embed default_prompt.txt) with
  llm.system_prompt / llm.system_prompt_file overrides; guided JSON parsing
  tolerant of thinking blocks
- address: recognize ordinals appearing after the street name
- tests: first coverage for the LLM adapter and config prompt loading; README

Measured (sample 200): deterministic 94.7-95.8% exact / 0 wrong across 4 seeds;
LLM+guard typo 90.7-89.8% / 0 wrong. LLM fallback latency ~1.2-1.9s.

internal/config/config_test.go

@@ -0,0 +1,69 @@
+package config
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func writeFile(t *testing.T, dir, name, content string) string {
+	t.Helper()
+	path := filepath.Join(dir, name)
+	if err := os.WriteFile(path, []byte(content), 0o600); err != nil {
+		t.Fatalf("write %s: %v", name, err)
+	}
+	return path
+}
+
+func baseConfig(promptFields string) string {
+	return `{
+      "addr": ":6171",
+      "data_path": "registry.db",
+      "portal": { "url": "http://portal" },
+      "llm": { "enabled": true, "url": "http://llm"` + promptFields + ` }
+    }`
+}
+
+func TestLoadInlineSystemPrompt(t *testing.T) {
+	dir := t.TempDir()
+	path := writeFile(t, dir, "config.json", baseConfig(`, "system_prompt": "INLINE PROMPT"`))
+
+	cfg, err := Load(path)
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if cfg.LLM.SystemPrompt != "INLINE PROMPT" {
+		t.Errorf("SystemPrompt = %q", cfg.LLM.SystemPrompt)
+	}
+}
+
+func TestLoadSystemPromptFromFile(t *testing.T) {
+	dir := t.TempDir()
+	promptFile := writeFile(t, dir, "prompt.txt", "PROMPT FROM FILE\n")
+	path := writeFile(t, dir, "config.json", baseConfig(`, "system_prompt_file": "`+promptFile+`"`))
+
+	cfg, err := Load(path)
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if cfg.LLM.SystemPrompt != "PROMPT FROM FILE\n" {
+		t.Errorf("SystemPrompt = %q", cfg.LLM.SystemPrompt)
+	}
+}
+
+func TestLoadMissingSystemPromptFileFailsClosed(t *testing.T) {
+	dir := t.TempDir()
+	path := writeFile(t, dir, "config.json", baseConfig(`, "system_prompt_file": "`+filepath.Join(dir, "nope.txt")+`"`))
+
+	if _, err := Load(path); err == nil {
+		t.Fatal("expected error for missing system_prompt_file")
+	}
+}
+
+func TestLoadRequiresPortalURL(t *testing.T) {
+	dir := t.TempDir()
+	path := writeFile(t, dir, "config.json", `{"addr": ":6171", "data_path": "x.db"}`)
+	if _, err := Load(path); err == nil {
+		t.Fatal("expected error for missing portal.url")
+	}
+}