fix(decoder): additional assert_malformed improvements

[avrabe]

Problem

Several assert_malformed tests fail because the decoder doesn't detect specific malformed conditions. ~35 total failures.

Specific gaps

Category	Count	Description
data count section required	4	Code section uses data.drop/memory.init but no data count section exists
malformed limits flags	4	Limits flags byte must be 0x00-0x03 (or 0x04-0x07 with memory64)
integer too large	9	LEB128 overflow not detected in some contexts
integer representation too long	3	LEB128 encoding uses more bytes than necessary
too many locals	4	Function declares more locals than allowed
unexpected end of section	4	Section contents shorter than declared size
section size mismatch	2	Section size doesn't match content
malformed reference type	2	Invalid reference type encoding
malformed memop flags	2	Invalid memory operation flags
illegal opcode	3	Undefined opcodes (e.g., 0xFF)
END opcode expected	2	Missing end opcode
malformed mutability	1	Invalid mutability flag

Files

• kiln-decoder/src/streaming_decoder.rs — main decoder
• kiln-build-core/src/wast_validator.rs — some checks may belong in validator