[rasros]

Fixes found in the 1.4.0 release audit. These bugs were already in v1.3.0. Encodings stay byte-identical to main.

• Decoding could exhaust memory from a 5-byte input ([FF FF FF FF 07] as List<Unit>). The fix caps declared collection elements per decode at 8 × input bytes + 2^20, so nested lists can't multiply the limit.
• Map.Entry didn't roundtrip: MapEntrySerializer opens a MAP via beginStructure, so no size was written. Such collections now write their own size.
• A Boolean registered as a polymorphic subclass threw IllegalStateException. Polymorphic wrappers now give the subclass payload its own child encoder/decoder.
• A huge string length could overflow the bounds check in decodeString.
• README: states that the wire format is only stable between patch versions.

lintDocs check passes on JVM, JS, wasmJs, wasmWasi and linuxX64.