Release rodbus 1.6.0-M1 (crates.io only)

[jadamcrain]

Milestone release publishing only the rodbus Rust crate to crates.io for the non-spawning server task APIs added in #188.

What's in it

• Full lockstep version bump 1.5.0 → 1.6.0-M1 across all crates, the FFI binding build files (CMake/pom/csproj), and the guide (sitedata.json). Cargo.lock regenerated by cargo.
• CHANGELOG.md entry for the new non-spawning create_tcp_server_task(), create_rtu_server_task(), create_tls_server_task(), and create_tls_server_task_with_authz() APIs.
• Stop tracking generated CycloneDX SBOMs (*.cdx.json) — see below.

Why the full bump for a crates.io-only release

The change is pure Rust with no FFI changes, but bumping rodbus alone fails CI. The FFI layer asserts the generated bindings' expected version (rodbus-schema, ffi/rodbus-schema/src/lib.rs) against the native library's runtime version (rodbus::VERSION, via ffi/rodbus-ffi/src/lib.rs); a lone rodbus bump makes those disagree and aborts the binding tests. So every crate moves together to keep the versions internally consistent.

SBOM cleanup

The 8 committed *.cdx.json files are build artifacts regenerated by cargo-cyclonedx in CI (the packaging job) and local runs — nothing consumes the committed copies. Being versioned, they drifted stale (1.5.0, carrying a colliding third-party lazy_static@1.5.0). This PR git rm --cacheds them and gitignores *.cdx.json.

Not hand-edited

Lockfiles (Cargo.lock, guide/package-lock.json) regenerate themselves; package-lock.json needs no change (its only 1.5.0s are unrelated npm deps).

Release process (differs from RELEASING.md)

This does not use the tag-driven pipeline (a tag fires all release jobs across every ecosystem). After merge, publish manually from main:

git checkout main && git pull
cargo publish -p rodbus --dry-run   # verify
cargo publish -p rodbus

No git tag is pushed — that would trigger the full Maven/NuGet/docs pipeline. Binding versions therefore read 1.6.0-M1 in source but stay published at 1.5.0 until the 1.6.0 final re-syncs everything.