refactor: rename socket_path to endpoint in AimDB MCP tools

[lxsaah]

Transport-agnostic host client — pick the transport at runtime (Issue #123)

Follow-up to #39 / #122. The host-side toolset (aimdb-client, the aimdb CLI,
and aimdb-mcp) no longer hard-codes the Unix-domain socket transport. A target
is now named by an endpoint — a scheme:// URL (or a bare path) — and the
scheme picks the transport at connect time, the same way a record's link URL
already picks one. Adding a transport is opt-in via a Cargo feature.

# Unix-domain socket (default transport)
aimdb --connect unix:///tmp/aimdb.sock record list
aimdb --connect /tmp/aimdb.sock record list          # bare path = unix:// shorthand

# Serial / UART (requires building with --features transport-serial)
aimdb --features transport-serial \
      --connect serial:///dev/ttyACM0?baud=115200 record list

# Falls back to AIMDB_CONNECT, then UDS auto-discovery
AIMDB_CONNECT=unix:///tmp/aimdb.sock aimdb record list
aimdb record list                                    # auto-discovery

What changed

aimdb-core

• impl Dialer for Box<dyn Dialer> — a boxed dialer is itself a Dialer, so a
  runtime-selected Box<dyn Dialer> (the resolver's return type) can be handed
  straight to run_client<D: Dialer> without a generic transport at the call site.

aimdb-client

• New endpoint module, split into two deliberately separate layers:
  • parse_endpoint(&str) -> ParsedEndpoint — pure, feature-independent grammar.
    Recognizes unix:// / uds:// / serial://DEVICE?baud=N and a bare path
    (the unix:// shorthand). An unknown scheme (e.g. tcp://) is rejected here.
  • dial(&str) -> Box<dyn Dialer> — builds the concrete transport under the
    matching transport-* feature. A scheme whose transport isn't compiled in is
    rejected here, with a clear error distinct from "unknown scheme".
• AimxConnection::connect now takes a &str endpoint (was impl AsRef<Path>),
  resolved through the endpoint module. New connect_over(dialer) /
  connect_over_with_timeout dial over an explicit Dialer, bypassing resolution.
• Transports are opt-in features: transport-uds (default) gates
  aimdb-uds-connector and the discovery module (a Unix-socket scan);
  transport-serial (off by default) gates aimdb-serial-connector
  (tokio-serial → libudev).
• New ClientError::UnsupportedEndpoint; ConnectionFailed.socket → endpoint;
  discovery::InstanceInfo.socket_path → endpoint.

aimdb CLI

• Per-command --socket <path> flags replaced by a global --connect <endpoint>
  (+ AIMDB_CONNECT env). Precedence: --connect → AIMDB_CONNECT → auto-discovery.
• instance info / ping now work over any endpoint; instance list stays
  discovery-only. New transport-serial feature wires serial:// into the resolver.

aimdb-mcp

• Every tool's socket_path parameter renamed endpoint and now accepts any
  endpoint URL. Startup --socket → --connect; AIMDB_SOCKET → AIMDB_CONNECT.
• Connection pool re-keyed by endpoint URL; public-mode SSRF stripping strips
  endpoint. get_instance_info's result field socket_path → endpoint
  (discover_instances keeps socket_path). New transport-serial feature.

Examples — worked demonstrations of --connect

The connector examples now stand up a remote-access server so the new client
toolset has something real to talk to, exercising both transports end-to-end:

Example	Transport	How to reach it
tokio-knx-connector-demo	UDS (UdsServer)	aimdb --connect unix:///tmp/aimdb-knx.sock record list
embassy-knx-connector-demo	Serial (SerialServer, USART3 / ST-LINK VCP)	aimdb --features transport-serial --connect serial:///dev/ttyACM0?baud=115200 record list
embassy-mqtt-connector-demo	Serial (SerialServer, USART3 / ST-LINK VCP)	same as above

• The remote-access server is registered as a second connector alongside the
  existing KNX/MQTT connector (the builder drives any number of them).
• All records are exposed read-only: the KNX/MQTT connector (or a sensor
  source) is the single writer for each key, so remote record.set is refused —
  this preserves AimDB's single-writer-per-key invariant.
• The shared knx-/mqtt-connector-demo-common crates gained a no_std-capable
  serde feature (serde derive was previously gated on std), so the embassy
  demos can serialize the real demo records over serial — not a stand-in.
• The embassy heap was bumped 32 → 48 KB to give the AimX server's JSON path room
  alongside embassy-net.

Breaking changes

• AimxConnection::connect / connect_with_timeout take &str (was a path).
• ClientError::ConnectionFailed.socket → endpoint;
  discovery::InstanceInfo.socket_path → endpoint.
• CLI: per-command --socket removed; use the global --connect (bare paths still work).
• MCP: tools' socket_path → endpoint; --socket → --connect;
  AIMDB_SOCKET → AIMDB_CONNECT; get_instance_info result socket_path → endpoint.
• Serial endpoints require building the CLI/MCP with --features transport-serial.

Testing

• aimdb-client tests pass in both feature configs (default transport-uds, and
  --no-default-features --features transport-serial); the UDS integration tests
  ride #![cfg(feature = "transport-uds")].
• Clippy clean (-D warnings) on aimdb-client / aimdb-cli / aimdb-mcp for
  both the default and serial-transport feature arms (Makefile parity).
• tokio-knx-connector-demo builds and was smoke-tested end-to-end: the UDS
  server comes up even with the KNX gateway unreachable, and against it the CLI
  resolved a bare path, a unix:// URL, and AIMDB_CONNECT; instance ping
  reported the endpoint; record set was correctly refused (read-only).
• Both embassy demos cross-compile to thumbv7em-none-eabihf; the tokio demos
  still build after the shared-crate serde change.