Seven new architectures: 386, arm, riscv64, ppc64/ppc64le, s390x, loo…

…ng64, and wasm

- Instruction decoding, operand normalization, and cross-build masking
  for each new architecture, with relocation-only fast paths on 386,
  arm, riscv64, and loong64.
- The amd64 fast path now verifies rip-relative references to text
  symbols, so a retargeted function reference is no longer classified
  as relocation noise.
- --mask-sp is arch-aware and --blocks splits blocks on every
  architecture.
- Loaders hardened against malformed ELF, Mach-O, PE, and wasm inputs;
  non-thread-safe x/arch decoders are serialized.
- mips remains unsupported pending an upstream golang.org/x/arch
  disassembler (golang/go#19158).

ixdiff: trim README intro and show a side-by-side example

Drop the implementation-detail paragraph from the introduction and
demonstrate --side-by-side with the same function as the unified diff
example, captured from the real tsgo comparison.