SFT-7490: fail closed on malformed peer frames

[georgesFoundation]

Malformed pool input could panic the client, wedge it permanently, or poison every frame behind it. Validate lengths and domains before slicing, shifting or resizing, and consume a frame before reporting its error.

Panics, all reachable from peer input:

• mining.notify sliced prev_hash at fixed offsets without checking its length.
• roll() shifted by 0i32.trailing_zeros(), i.e. 32, when the pool granted a zero version mask, and added ntime_bits to a pool-supplied ntime without wrapping.
• set_extranonces assigned extranonce2_size before the resize that could fail, leaving it out of sync with the buffer roll() indexes.
• send() wrote the terminator at tx_buf[len] without checking there was room for it.
• serde-json-core's custom-error-messages formats into a heapless::String<64> with write!(..).unwrap(), and serde's Display impls ignore the {:.64} precision, so any message over 64 bytes panicked. A response carrying the standard "jsonrpc" field is enough to reach it, so this fired against well-behaved pools too. The feature is now off and those errors carry no message.

poll_message returned parser and unknown-id errors before advancing past the offending line, so it stayed at the head of the buffer and every later poll failed identically. A buffer filled without a terminator read into an empty slice forever; it is now dropped with LineTooLong and the client resynchronizes on the next terminator.

Server-selected sizes are bounded before they reach an allocation or a resize. The new limits module documents the maxima; they are the capacities the heapless build already gets from its field types, so both builds now reject the same frames with the same errors.

Adds a mock-transport integration suite covering short hashes, unknown ids, invalid-then-valid JSON, fragmented reads, full buffers and oversized server sizes; a stateful fuzz target over structured frames, since a byte-level fuzzer never discovers "mining.notify" and would never get past parse_method; and a flake check for the alloc build, which the all-features checks have to exclude and which was therefore untested.

[badicsalex]

+1100 lines :/