Raise COINBASE_TYPE_YUGE to 64000 bytes and the coinbaser output limit to 2048

[iohzrd]

Based on #28, which is based on #24. Review the last commit. This is an alternative to #29: merge one of the two.

Needs #26 on musl. datum_protocol_pow keeps 140528 bytes on the stack, and assembleBlockAndSubmit keeps 133280 (gcc 16.2 -fstack-usage, x86_64, default CMake build). Both are more than musl's 128 KiB default thread stack.

On BLAKE2b work every miner is served COINBASE_TYPE_YUGE, and the coinbase never reaches the miner, so no firmware limit applies to its size. This raises it. Fingerprinting and classes 1, 2, 3 and 5 are unchanged.

• MAX_DICTATED_COINBASE_SIZE is 64000 (YUGE was 16000). That holds 2048 P2WPKH outputs or about 1480 taproot outputs.
• datum_stratum_coinbase_fit_to_template still cuts the coinbase to the template's room. At the node's default -blockreservedweight (8000 WU) that is about 1800 bytes on a full block. Filling 64000 bytes needs about 257000 WU, 32% of an 800000 WU RDTS block.
• MAX_COINBASER_OUTPUTS is 2048 (was 512). It sets the coinbaser parse limit and the size of available_coinbase_outputs.
• STRATUM_COINBASE2_MAX_LEN is 131072 hex characters (was 32768), so coinb2 can be up to 65535 bytes. The share's coinbase section writes each part's length as a uint16.
• MAX_POW_MESSAGE_SIZE (MAX_COINBASE_TXN_SIZE_BYTES + 4096) replaces 32768 as the size of datum_protocol_pow's message buffers and as the replay limit.
• DATUM_PROTOCOL_COINBASER_BUFFER_SIZE is 65536 (was 32768) for the coinbaser response, whose length field is a uint32. A response is one byte plus its outputs encoded as in the coinbase, so it holds every dictated output a 64000-byte coinbase has room for.
• Test: 2048 P2WPKH outputs fit one coinbase, with a three-byte output count.

64000 is below the protocol's limit for this layout, 65610 bytes: coinb2 at 65535, the 63-byte coinb1 of a DATUM job with no tags, and the 12-byte extranonce. The largest share carrying a 64000-byte coinbase is 65372 bytes (encrypted, with its pad), so it fits a 64 KiB frame.

The pool server has to accept a share coinbase section (coinb1 + coinb2) of up to 63988 bytes, a share message of up to 65372 bytes, and a coinbaser response of up to 65535 bytes.

BSS goes from 89.5 MB to 212.3 MB.

The commit also reformats datum_stratum.h: Allman braces, and spaces for alignment.

Every commit builds with -Wall -Werror and passes --test.