perf: Reduce generated code overhead

[dcodeIO]

Reduces generated-code overhead from recently merged work while preserving wire-format behavior.

• Moves presence checks introduced in fix: Omit proto3 implicit scalar defaults #2208 from encoders to converters and decoders. This is cheaper because converters can avoid materializing implicit scalar defaults in the first place, and decoded values have already been normalized to their runtime representation.
• Partly reverts fix: Decode fields by full wire tag #2197 to dispatch by field number again, using compact wire-type guards instead, to retain the runtime benefit of dense switch cases.
• Avoids redundant constructor calls during map message value allocation introduced in fix: Decode missing map message values as empty messages #2206.

Also refreshes affected fixtures and adjusts tests for implicit scalar default elision during conversion.

I also experimented with additional codegen micro-optimizations, but they did not appear worthwhile relative to the generator's code size increase.