perf(decode): cache struct schemas per type

Assisted-by: GLM 5.3

docs/ARCHITECTURE.md

@@ -101,9 +101,15 @@ sequenceDiagram
   setter methods are not, and must finish before the value is shared.
 - The parser is allocated per `ParseContext` call; nothing is cached between
   documents.
+- The one piece of shared state is the struct-schema cache in `decode.go`: a
+  `sync.Map` keyed by `reflect.Type`, holding the flattened field layout the
+  decoder and the encoder both consult. A schema is immutable once published,
+  so concurrent callers only race to build an identical value, the same
+  trade-off `encoding/json`'s field cache makes. The cache grows with the
+  number of distinct struct types, never with document size.
 - The date-time wrappers are values, not pointers, and are immutable in use.
-- Nothing in the library starts goroutines or holds locks; concurrency safety
-  comes from having no shared mutable state.
+- Nothing in the library starts goroutines; apart from the schema cache above,
+  which never mutates a published entry, there is no shared mutable state.
 
 ## Dependencies