De-genericize HTTP handlers via T-free node modules; add AdminClient

[antondalgren]

Summary

`StatusHandler(T)` / `AdminHandler(T)` were generic only because their field was typed `Node(T)` — every method they call is T-free. The genericity forced LavinMQ to wrap them in a concrete class (`RaftHandlerWrapper`) to dodge a Crystal codegen bug: stdlib `HTTP::Handler#call_next` builds a type-id case table over concrete includers; generic instantiations are missing from it, so a generic handler in a chain traps with `brk #1`.

Changes

1. `Raft::StatusSource` / `Raft::AdminOps` — T-free abstract modules, at node level (not `http/`): `Node` includes them, and core requiring `http/` would invert the ARCHITECTURE.md layering.
2. `Node(T)` includes both + four new T-free log scalars (`last_log_index`, `last_log_term`, `first_log_index`, `segment_count`) so status consumers stop reaching through the mutable `Node#log`.
3. Handlers drop `(T)` — fields are typed by capability and named for it too (`@source : StatusSource`, `@ops : AdminOps`), since future non-node implementors (e.g. a channel-marshaling admin wrapper) can include the same modules. Behavior byte-for-byte identical.
4. `Raft::HTTP::AdminClient.add_server(uri, node_id, address)` — client side of the join wire format. Single attempt, no retry loop (embedders own retry policy). Returns `::HTTP::Status` instead of Bool so retry policies can distinguish rejection (400 → stop) from unavailability (5xx → retry). Basic auth from URI userinfo; URI path prefix respected.

What this is NOT

A typing change only — no change to who may call the node or from where. The single-fiber threading contract is restated on both modules. The thread-safety marshaling discussion (AsyncAdmin) is deliberately out of scope; an `AsyncAdmin` would `include AdminOps`, so nothing here forecloses it.

LavinMQ after this

`RaftHandlerWrapper` can be deleted — handlers are concrete and chain-safe directly. The join flow can use `AdminClient.add_server` instead of hand-rolled HTTP.

Test plan

• `crystal spec spec/raft/ -Dpreview_mt -Dexecution_context` — 124/124 (was 118; +6 AdminClient)
• `crystal spec … -Draft_debug` — 131/131
• Chain-parity spec (both handlers mounted together, both surfaces served) passes
• AdminClient round-trip against real AdminHandler: peer actually added
• `crystal tool format --check` clean; ameba `--fail-level error` exit 0 with no new warnings
• All new/touched files compile standalone with `--error-on-warnings`; both examples compile

🤖 Generated with Claude Code