Add YAML configuration model and loader

Config is layered: built-in defaults, then the YAML file, then BFD_* environment variables, then CLI flags. Global defaults live under a 'defaults' mapping; per-peer settings are file-only. Includes config.yaml.dist and ScriptHookSink::with_script_dir so the hooks directory is configurable.

config.yaml.dist

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+# rust-bfd configuration. Install as /etc/rust-bfd/config.yaml (or pass --config / set BFD_CONFIG).
+#
+# Precedence, lowest to highest: built-in defaults < this file < BFD_* environment < command line.
+# Values shown below are the built-in defaults unless noted. Unknown keys are an error.
+
+# Local address(es) to bind and use as the source of BFD control packets. Required (here, in BFD_BIND, or via --bind).
+# Accepts a single address, a list, or a comma-separated string. The daemon currently supports exactly one address;
+# the list form is accepted so configs do not need to change once multiple binds are implemented.
+#   Env:  BFD_BIND="192.0.2.1"            (or "192.0.2.1,2001:db8::1")
+#   CLI:  --bind 192.0.2.1 [--bind ...]   (replaces the addresses configured here)
+bind: 192.0.2.1
+# bind: [192.0.2.1, "2001:db8::1"]
+
+# Directory of executable scripts run, in lexical order, on every session state change.
+#   Env: BFD_HOOKS_DIR
+hooks_dir: /etc/rust-bfd/hooks.d
+
+# Settings inherited by every peer that does not override them. Durations are humantime strings (300ms, 1s, 2m).
+# A peer is never allowed a zero interval or a detect_mult of 0.
+defaults:
+  # Minimum interval between transmitted control packets that we would like to use once the session is Up. Sessions
+  # that are not Up transmit no faster than once per second regardless (RFC 5880 section 6.8.3).
+  #   Env: BFD_DEFAULTS_DESIRED_MIN_TX
+  desired_min_tx: 300ms
+  # Minimum interval between received control packets we can support.
+  #   Env: BFD_DEFAULTS_REQUIRED_MIN_RX
+  required_min_rx: 300ms
+  # Detection time multiplier: we declare the session down after this many missed packets.
+  #   Env: BFD_DEFAULTS_DETECT_MULT
+  detect_mult: 3
+
+# Peers to establish sessions with. Peers can only be configured here and with --peer (repeatable, addresses only,
+# default settings); they cannot be set from the environment. Each peer's address must match the family of at least
+# one bind address.
+peers:
+  - address: 192.0.2.2
+  # Per-peer overrides of any `defaults` key:
+  # - address: 192.0.2.3
+  #   desired_min_tx: 1s
+  #   required_min_rx: 1s
+  #   detect_mult: 5