v2.0.3

v2.0.3 — CI release-attach + Linux daemon depends/

No source / consensus changes since v2.0.2.

CI infrastructure only:
- Added linux-build-depends.yml: static-linked Linux x86_64 daemon
  via depends/ (no system Boost runtime dependency — fixes the
  libboost-version-mismatch complaint).
- Removed windows-build.yml (MSYS2 native): parallel-insurance Windows
  path was chasing MSYS2 toolchain drift with no shipping value.
  depends/ cross-compile (windows-build-depends.yml) remains canonical.
- Linux + Windows workflows now auto-attach their tarballs to the
  GitHub Releases page on tag push instead of requiring manual upload.
- Per-platform SHA256SUMS-{linux,windows}.txt files so the release
  page shows checksums for both platforms (single SHA256SUMS would
  collide).

This release validates the auto-attach pipeline. Binaries land in a
draft release; publish manually after verifying SHA256SUMS-*.txt.