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>

document_test.go

@@ -195,6 +195,7 @@ func TestDocumentRefusesAnEditItCannotPlace(t *testing.T) {
 // TestUnmappedNamesWhatDecodeIgnores: keys no field takes, by tag or field name,
 // unexported and "-" fields included, are named once each, under arrays of tables
 // without their index; a map or interface destination takes every key under it.
+// A parsed tree is read as its document is, and UnknownKeys quotes each.
 func TestUnmappedNamesWhatDecodeIgnores(t *testing.T) {
 	doc := `top = 1
 skip = 2
@@ -248,6 +249,12 @@ nope = 1
 	if !slices.Equal(got, want) {
 		t.Fatalf("unmapped %q, want %q", got, want)
 	}
+	if tree := Unmapped(d.Tree(), &v); !slices.EqualFunc(tree, d.Unmapped(&v), slices.Equal) {
+		t.Fatalf("the tree's unmapped %q differ from the document's", tree)
+	}
+	if err := UnknownKeys(d.Unmapped(&v)[:2]).Error(); err != `unknown keys "byname.one.nope", "hidden"` {
+		t.Fatalf("error %s", err)
+	}
 }
 
 // TestKeyStringQuotesWhatABareKeyCannotHold