Implement ManagedImage and codec abstraction (#38)

README.md

@@ -122,5 +122,12 @@ five parsers without an external fuzzing runtime. They cover truncation, tags,
 lengths, payloads, delimiters, nesting, entity rejection, contextual errors,
 and the parser resource limits. Every encoder sorts map keys where necessary,
 so serialization is deterministic across platforms.
+Milestone 06 starts with a native `ManagedImage`: top-left-origin,
+byte-per-sample planar storage has an explicit width-byte row stride, with gray
+stored in the red plane and optional color, alpha, and bump planes. Checked
+constructors and resizers cap decoded images at 4096 by 4096 worth of pixels,
+codec adapters cap buffered encoded input at 64 MiB, and canonical
+interleaving accepts explicit row strides without exposing codec-library types
+through the core abstraction.
 The controlled audit aggregates every expected failure by standardized C#
 member ID and rejects unrelated fixture, assertion, compile, or symbol errors.