feat(fuzz): add SharpFuzz-based fuzzing for untrusted-input parsers

New tests/PalladiumWallet.Fuzz project: one target per parser that
consumes untrusted input (block headers, Merkle proofs, peer lists,
wallet files, user-pasted mnemonics/keys/addresses/amounts), each
encoding the parser's documented error contract - any exception beyond
that contract is a finding, reported as a crash.

Three ways to run: the seed corpus (with regression inputs for every
crash found so far, including the two fixed in prior commits) replays
automatically inside dotnet test via FuzzCorpusTests, so a fixed
contract violation can't silently return; a built-in random-mutation
mode needs no external tooling (dotnet run -- <target> --random N);
and fuzz.sh drives real coverage-guided campaigns via afl++ +
SharpFuzz instrumentation for pre-release or post-parser-change runs.

tests/PalladiumWallet.Fuzz/Corpus/peers/odd-shapes.bin

@@ -0,0 +1 @@
+[[1,2,3],{"a":1},"x",[["h","n",[42,"t"]]]]

tests/PalladiumWallet.Fuzz/Corpus/peers/regression-bad-utf8.bin

@@ -0,0 +1 @@
+[["1.2.3.4","peer.example",["v1.4.2","p10","t50001","‡50002"]]]

tests/PalladiumWallet.Fuzz/Corpus/peers/typical.bin

@@ -0,0 +1 @@
+[["1.2.3.4","peer.example",["v1.4.2","p10","t50001","s50002"]]]