Fix/ttl dscp portable sockopt constants

[kristapssk]

Description

Two IP-level socket options in crates/hdds/src/transport/ used hardcoded
Linux numeric constants that are different on macOS/BSD, causing runtime
failure with IoError(Os { code: 42, kind: Uncategorized, message: "Protocol not available" })
(ENOPROTOOPT) whenever a Participant tried to configure IP TTL, multicast TTL,
or DSCP on Apple platforms.

Fix: replace the hardcoded numbers with the corresponding libc::* constants
so the correct per-platform value is resolved at compile time.

Constant	Linux (old hardcoded)	macOS/BSD	Now
IP_MULTICAST_TTL	33	10	libc::IP_MULTICAST_TTL
IP_TTL	2	4	libc::IP_TTL
IP_TOS	1	3	libc::IP_TOS
IPPROTO_IP	0	0	libc::IPPROTO_IP

Linux behavior is byte-for-byte unchanged (the libc::* values resolve to
the same numeric constants). Windows paths already went through socket2
and are untouched.

Files:

• crates/hdds/src/transport/ttl.rs
• crates/hdds/src/transport/dscp.rs

Related Issue

N/A — no upstream issue was open. Discovered while building a downstream
TransportMode::UdpMulticast publisher on macOS 15 (Apple Silicon).

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Checklist

• My code follows the style guidelines of this project (cargo fmt --all -- --check passes)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas (existing // SAFETY: blocks preserved verbatim)
• I have made corresponding changes to the documentation (no public API changed)
• My changes generate no new warnings (cargo clippy on touched files is clean)
• I have added tests that prove my fix is effective or that my feature works (existing unit tests already exercise both code paths; they now pass on macOS as well as Linux)
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules (none required)
• Commits are DCO-signed (Signed-off-by: trailer on every commit)

Testing

Linux (containerized rust:latest, aarch64):

docker run --rm -e CARGO_TARGET_DIR=/tmp/tgt -v "$PWD":/work -w /work rust:latest bash -c "
  cargo fmt --all -- --check &&
  cargo test -p hdds --lib transport::ttl &&
  cargo test -p hdds --lib transport::dscp
"
# Result: 7/7 ttl + 5/5 dscp = 12/12 passed