Feature/topology loader

[ankenman]

Add JSON topology loading

Introduces TopologyLoader to construct and wire SoC modules from a JSON
file, replacing the hardcoded topology in main.cpp.

Topology file format:

• modules: array of {name, type, id, neighbors, config}
• module types: initiator, home_agent, target, interconnect
• neighbors: list of module names this module connects to (symmetric;
  loader dedupes when the same edge appears in both endpoints' lists)
• config: per-module knob overrides

The loader:

• Parses JSON, validates schema and module types
• Constructs each module via a type-specific factory method
• Applies per-module config blocks to the knob system
• Wires neighbors by allocating ports keyed by neighbor id and binding
  the resulting port pairs

Module refactoring for topology-driven construction:

• Initiator, HomeAgent, Target: constructor takes only (sim, sys, id,
  name); previously type-specific args like clock_period_ps,
  home_id, downstream_target_id, and data_latency_cycles. These are now
  knobs, captured in elaborate().
• WorkQueues and TransactionQueue (which depend on clock_period_ps or
  tq_capacity) become unique_ptr members, constructed in elaborate()
  after knobs are finalized.
• Port management moves to Module base: each module owns a port_map
  keyed by neighbor id, via add_port(neighbor_id) and get_port(...).
  Single-port agents use single_port() helper.
• Callback registration (on_receive) moves to elaborate(), running
  after the loader has allocated ports.
• Interconnect's attach() removed; the loader's wire_neighbors handles
  what attach used to do.

Port: store peer as a pointer instead of snapshotting the callback at
bind time. This allows on_receive to be registered after bind, which
the new construction order requires.

Main.cpp: phases reduced to parse_command_line (non-strict) →
loader.load(topology_file) → check_for_help → parse_command_line
(strict) → elaborate_all → tracer setup → start_all → run.

Adds --topology command-line flag handled in csim::config
alongside --config and --json.

Tests: topology_loader_test covers smoke, construction, config block
application, neighbor wiring, and error cases (duplicate names,
unknown types, unknown neighbors).