Generate specialized encoder functions at compile time (3-6x faster encoding)

[csWen]

Implements the proposal from #448.

What changes

Protobuf.encode/1 currently interprets MessageProps on every call: per-field dispatch on metadata, presence checks, and IO.iodata_length/1 walks to write length prefixes. This PR moves that work to compile time: the DSL compiles each message into a specialized __encode_sized__/1 that returns {iodata, byte_size}, so dispatch, presence checks, tags, and wire types are resolved once at compile time and parents write length prefixes without re-walking the iodata. Map entries are encoded straight from key/value pairs, without building entry structs.

The interpreter stays for what isn't worth specializing: messages with a transform_module, values the generated clauses reject, and error reporting — a failing message is replayed through the interpreter so Protobuf.EncodeError names the failing field exactly as before.

Varint.encode/1 now builds each varint as a single binary in one bit-syntax instruction, and rejects integers that don't fit in 64 bits instead of silently truncating them.

Results

Google benchmark datasets (Apple M1 Max, Elixir 1.18.2 / OTP 26):

dataset	main	this PR	speedup	memory
google_message1_proto2	6.40 µs	1.13 µs	5.7x	2.80 KB → 1.50 KB
google_message1_proto3	6.76 µs	1.37 µs	4.9x	2.59 KB → 1.37 KB
google_message2	2.80 µs	0.46 µs	6.1x	1.67 KB → 616 B

The production ~8 MB message from #448 goes from 716 ms to 223 ms (3.2x) — large messages sit at the lower end of the range since raw byte copying takes a bigger share of the time.

Costs

Every message module now carries its generated encoder: compiling this repo's test suite goes from ~3.8 s to ~8.9 s, and ebin grows from ~2.8 MB to ~3.8 MB. Decoding is untouched.

Compatibility

• Wire output is byte-identical to main; the conformance suite passes unchanged (2674 tests, 0 failures). All 581 unit tests, Dialyzer, and Credo pass.
• No public API changes: __encode_sized__/1 is a generated internal, everything still goes through Protobuf.encode/1.