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>

README.md

@@ -20,7 +20,10 @@ replace old containers. Existing pointer defaults are retained without writing
 through the original pointer. Fixed arrays require an exact input length.
 Tags use `toml:"name,omitempty"`; an empty tag name
 falls back to the Go field name. Matching is case sensitive. Unknown input keys
-and unexported fields are ignored.
+and unexported fields are ignored; `Unmapped(data, &target)` names the keys a
+parsed value holds that no field takes, and a caller refusing them returns
+`UnknownKeys(keys)`, which quotes each. A decode error names where it failed by
+the key path the document spells: `rooms[1].width: cannot convert string to int`.
 
 ## Values and conversions
 
@@ -121,7 +124,8 @@ split at the widest gap.
 - `Unmapped` names the keys no field of the destination takes, matched as `Decode`
   matches them (tags, then field names; unexported and `-` fields take nothing),
   each once and sorted. A map or interface destination takes every key under it;
-  a key under an array of tables is named without its index.
+  a key under an array of tables is named without its index. The function of the
+  same name reads any value `Parse` produced, a table inside one included.
 - `Key` is a dotted path, one segment per part; `Key.String` spells it as a
   document would, quoting segments that need it.