relay: bootstrap timeouts, conn cap, and topic set bounds

[intendednull]

Summary

Two relay-side DoS hardening fixes:

• Closes [relay] Bootstrap HTTP endpoint has no timeout or connection cap (Slowloris) #112 — crates/relay/src/main.rs bootstrap-id HTTP endpoint had no I/O timeout (Slowloris-friendly), no concurrent-connection cap (FD/memory exhaustion), and no Connection: close header. Wrap each per-connection read and write_all in tokio::time::timeout(BOOTSTRAP_IO_TIMEOUT = 5s), gate accepts on Arc<Semaphore> sized to MAX_CONCURRENT_BOOTSTRAP_CONNECTIONS = 1024 via try_acquire_owned() (held for the lifetime of the per-connection task), and add Connection: close to the response.
• Closes [relay] Topic subscription set grows without bound #113 — topic_announce_listener kept an unbounded HashSet<String> of subscribed topics, so any signed peer could announce arbitrary unique strings to grow the set forever and trigger an unbounded number of network.subscribe calls. Cap the set at MAX_TOPICS = 10_000 (drops further unique announces with one warn) and validate every incoming string via the new topic_str_is_valid helper (MAX_TOPIC_LEN = 256, ASCII alphanumerics + _ / : . -). Invalid topics are warn-logged and never inserted/subscribed.

The bootstrap handler and topic-announce listener moved into crates/relay/src/lib.rs (previously a stub) so they can be exercised by integration tests in crates/relay/tests/. main.rs is now a thin shim that wires them up. Nothing else in the workspace depends on willow-relay, so the new lib has no downstream impact.

Progresses #108.

Test plan

New regression coverage in crates/relay/tests/bootstrap_endpoint.rs plus topic_str_is_valid unit tests in crates/relay/src/lib.rs:

• topic_str_is_valid_accepts_basic_ascii — alphanumerics, /, _, :, ., - all accepted
• topic_str_is_valid_rejects_empty — empty string rejected
• topic_str_is_valid_rejects_too_long — MAX_TOPIC_LEN + 1 rejected, exact MAX_TOPIC_LEN accepted
• topic_str_is_valid_rejects_disallowed_chars — whitespace, control bytes, punctuation, non-ASCII, NUL all rejected
• bootstrap_endpoint_serves_normal_request_quickly — a normal GET returns the id within ms
• bootstrap_response_contains_connection_close_header — response carries the new header
• handle_bootstrap_connection_times_out_slow_reader — paused tokio time + tokio::io::duplex pipe; advancing past BOOTSTRAP_IO_TIMEOUT yields ErrorKind::TimedOut without 5s of wall clock
• bootstrap_listener_drops_connections_when_capacity_saturated — capacity-1 semaphore + a held connection forces the next accept to be denied a permit; the test asserts the second client sees EOF (or ConnectionReset) instead of a response
• bootstrap_listener_recovers_after_permit_released — once a connection completes its permit is released and the next request succeeds again
• cargo test -p willow-relay — all 10 tests pass (4 unit, 6 integration), under 0.1s wall clock
• cargo clippy --workspace --all-targets -- -D warnings — clean
• cargo fmt --check — clean

cargo test --workspace could not be run to completion in this worktree due to disk pressure from sibling worktrees, but cargo clippy --workspace --all-targets (which type-checks every test target) is clean and willow-relay is a leaf binary crate with no reverse dependencies.

https://claude.ai/code/session_018TTGhL645aTR4RZWrRBLPS