Add missing requires so each src file compiles standalone

[antondalgren]

Summary

Each `.cr` in `src/` now declares its own dependencies via `require`, so
```
crystal build --no-codegen --error-on-warnings -Dpreview_mt -Dexecution_context src/raft/http/handler.cr
```
(and the same for every other file) succeeds standalone, not only via the `src/raft.cr` aggregator. Mirrors what Carl's `29fb3fb` was originally trying to land but with the deps relevant to today's tree.

What was missing

• `node.cr` used `Role`, `NodeID`, `Log`, `StateMachine`, `Metrics`, `Peer`, `LogEntry`, `EntryType`, `Message`, `Config` without requiring any of them.
• `log.cr` / `log/segment.cr` / `log_entry.cr` didn't pull in their type dependencies (`Config`, `EntryType` from `message.cr`).
• `message.cr` / `peer.cr` / `metrics.cr` used the `NodeID` alias from `config.cr` without requiring it.
• `transport.cr` (abstract base) didn't require `config` / `message` for the types in its abstract signatures.
• `transport/tcp_transport.cr` and `transport/memory_transport.cr` didn't require `../transport` (their base class), `../config`, etc.
• `server.cr` used `Node`, `StateMachine`, etc. without requiring them.
• `http/handler.cr` used `TCPTransport` without requiring it.

Test plan

• `crystal build --no-codegen --error-on-warnings -Dpreview_mt -Dexecution_context ` — clean for all files (lib + tui)
• `crystal build --no-codegen ... src/raft.cr` (the aggregator) — still clean
• `crystal tool format --check src/ spec/` — clean
• `lib/ameba/bin/ameba --fail-level error src/ spec/` — clean
• `crystal spec spec/raft/ -Dpreview_mt -Dexecution_context` — 113/113

Why it matters

• LavinMQ's `compile-changed-files` CI job (which we may want to port) iterates each changed `.cr` and tries to build it standalone. With this PR, that pattern would work for raft.cr.
• Editor tooling (LSP, jump-to-def) often resolves `require` graphs per-file; explicit requires make each file's dependencies obvious to both humans and tools.
• It prevents a class of "works in CI, doesn't work when someone reorders the aggregator" surprises.

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