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.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"reflect"
 	"slices"
+	"strconv"
 	"strings"
 )
 
@@ -253,12 +254,30 @@ func (d *Document) Tree() map[string]any { return d.tree }
 func (d *Document) Decode(v any) error { return Decode(d.tree, v) }
 
 // Unmapped names the keys of the document that no field of v's type takes, as
-// Decode, which ignores them, would match them; each once and sorted. A key under
-// a map or interface destination is taken, and one under an array of tables is
-// named without its index.
-func (d *Document) Unmapped(v any) []Key {
+// the function Unmapped does
+func (d *Document) Unmapped(v any) []Key { return Unmapped(d.tree, v) }
+
+// UnknownKeys is the error of a caller that refuses the keys Unmapped names
+type UnknownKeys []Key
+
+func (u UnknownKeys) Error() string {
+	q := make([]string, len(u))
+	for i, k := range u {
+		q[i] = strconv.Quote(k.String())
+	}
+	if len(q) == 1 {
+		return "unknown key " + q[0]
+	}
+	return "unknown keys " + strings.Join(q, ", ")
+}
+
+// Unmapped names the keys of data, a value Parse produced, that no field of v's
+// type takes, as Decode, which ignores them, would match them; each once and
+// sorted. A key under a map or interface destination is taken, and one under an
+// array of tables is named without its index.
+func Unmapped(data any, v any) []Key {
 	seen := make(map[string]Key)
-	unmapped(d.tree, reflect.TypeOf(v), nil, seen, 0)
+	unmapped(data, reflect.TypeOf(v), nil, seen, 0)
 	out := make([]Key, 0, len(seen))
 	for _, k := range seen {
 		out = append(out, k)