v0.5.0 Unmapped reads any parsed value, UnknownKeys refuses what it names, decode errors name the key path

* Unmapped reads any parsed value; decode errors name the key path

Unmapped(data, v) names the keys of a value Parse produced that no field
of v's type takes, a table inside a document included, so a caller holding
a tree (after macro expansion, say) can refuse them as a Document's caller
does; Document.Unmapped calls it.

A decode error names where it failed by the key path the document spells,
rooms[1].width or by_key."a b".width, rather than by Go type and field
names and "index n" / "map key" segments; a key that cannot be bare, or
runs past 32 bytes, is quoted and cut as parse errors quote one.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Rsnp29663hDvPHPua37MhN

* UnknownKeys, the error naming the keys a caller refuses

A caller that refuses what Unmapped names returns UnknownKeys(keys),
"unknown key \"a\"" or "unknown keys \"a\", \"b.c\"", so every refusal
words them alike. vif's settings already format their own; its FSM
dialect would be the second copy.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Rsnp29663hDvPHPua37MhN

---------

Co-authored-by: Claude <noreply@anthropic.com>

audit_fix_test.go

@@ -303,6 +303,29 @@ func TestDecodeErrorPathIsLinear(t *testing.T) {
 	}
 }
 
+// A decode error names where it failed by the key path the document spells
+func TestDecodeErrorNamesItsKeyPath(t *testing.T) {
+	type room struct {
+		Width int `toml:"width"`
+	}
+	var v struct {
+		Play  struct{ Sound bool } `toml:"play"`
+		Rooms []room               `toml:"rooms"`
+		ByKey map[string]room      `toml:"by_key"`
+	}
+	for _, tt := range []struct {
+		doc, want string
+	}{
+		{"play.Sound = 1", "play.Sound: cannot convert int64 to bool"},
+		{"rooms = [{ width = 1 }, { width = \"x\" }]", "rooms[1].width: cannot convert string to int"},
+		{"by_key.\"a b\".width = true", `by_key."a b".width: cannot convert bool to int`},
+	} {
+		if err := Unmarshal([]byte(tt.doc+"\n"), &v); err == nil || err.Error() != tt.want {
+			t.Errorf("%s: error %v, want %s", tt.doc, err, tt.want)
+		}
+	}
+}
+
 // Header paths are bounded: deep nesting is written inline past the bound,
 // so output stays linear and still round-trips
 func TestMarshal_HeaderLengthBounded(t *testing.T) {