[RFC] IPv6 / dual-stack support (TCP control plane + RDMA data plane)

## Motivation

Some RDMA fabrics are IPv6-only: high-bandwidth NICs get ULA + SLAAC IPv6
addresses and no IPv4 at all. 3FS currently cannot run on such fabrics —
`Address` is IPv4-only by design, so neither the TCP control plane nor the
RDMA data plane can bind or dial IPv6 endpoints.

## Root cause

`src/common/utils/Address.h`:

- `uint32_t ip` + `static_assert(sizeof(Address) == sizeof(uint64_t))`;
- `operator uint64_t()` is used as the hash/equality/serialization primitive;
- `is_serde_copyable` makes RPC serialization a raw 8-byte memcpy.

Fitting a 16-byte IPv6 address requires reworking this whole chain, not a
one-line patch.

## What we have implemented and validated

We have a working dual-stack implementation (~19 files, +292/−87 against
current main), running for several weeks on a 2-node test deployment:

- `Address`: 8B → 20B (16-byte address + family), IPv6 literal parsing
(`TCP://[fd00::1]:8000`), FNV hash + explicit `operator==`, factory helpers.
- `IfAddrs`: collect `AF_INET6` addresses (skipping link-local).
- `Listener`: new `ipv6_only` option (default false, IPv4 deployments
unaffected); `ConnectionPool` / `TcpSocket` generalized via
`sockaddr_storage` with per-family branches.
- RDMA: `rdma_resolve_addr` with `sockaddr_in6`, dual-stack peer info in QP
metadata exchange, RoCE v2 IPv6 GID selection.
- Unit tests updated/added (`TestAddress` IPv6 cases, `TestEcho`,
`TestIBSocket`).

Validation: full cluster (mgmtd / meta / storage / FUSE) over IPv6 for both
TCP control plane and RDMA data plane; real LLM checkpoint loads through FUSE
at multi-GB/s per NIC; unit tests pass.

## The design question we want maintainer input on

We kept `is_serde_copyable` and simply memcpy 20 bytes. This is the smallest
change, but it makes the wire format incompatible with existing v4 binaries:
old and new nodes cannot coexist, so no rolling upgrade. Options we see:

1. Accept as a breaking change (flag day / major version bump) — simplest.
2. Move `Address` to field-based serde for cross-version compatibility —
cleaner long-term, touches RPC schema.
3. Version-negotiated dual format — most compatible, most complexity.

Which direction would you prefer? We are happy to adapt the implementation.

## Proposed contribution plan

If there is interest, we would split the work into reviewable PRs:

1. `Address` dual-stack core + unit tests;
2. TCP path (IfAddrs / Listener / ConnectionPool / TcpSocket);
3. RDMA path (IBConnect / IBSocket / GID selection).

Happy to share more benchmark details or rebase onto any preferred design.