feat: add cross-process rate-limit coordination for Semantic Scholar

[Burton-David]

What

AdaptiveRateLimiter is process-local, so a research-mcp repl next to Claude Desktop's research-mcp serve (plus an ad-hoc script) each respect S2's limit individually but collectively exceed it — the user sees more 429s than they should.

SharedAdaptiveRateLimiter (sources/_rate_limit_shared.py) is a drop-in for AdaptiveRateLimiter (same acquire / record_failure / record_success / current_interval) that coordinates through a JSON sidecar at ~/.cache/research-mcp/rate-limits/<source>.json, guarded by an fcntl lock:

• last_call and the adaptive interval are shared, so every process on the machine serializes against one minimum interval and one backoff (a 429 seen by one process slows the others).
• acquire() reserves its slot — writes the time it will fire — while holding the lock, then sleeps outside the lock. The blocking lock + IO runs in asyncio.to_thread, so a sibling process holding the lock can't stall the event loop.
• Wall-clock time.time() (comparable across processes, unlike the local limiter's monotonic); atomic tmp+rename writes; a missing/corrupt sidecar recovers to baseline.
• record_* stay synchronous (they're invoked as backoff callbacks) and best-effort — a lock/IO failure never raises into the request path.

Opt in for Semantic Scholar with RESEARCH_MCP_S2_SHARED_RATELIMIT=1 (default off, so the test suite never touches shared ~/.cache state). POSIX only — fcntl; Windows is deferred.

Closes #8.

Test plan

• ruff check src tests / mypy src — clean
• pytest -q --strict-markers — 537 passed, 14 skipped
• tests/unit/test_shared_rate_limit.py (all state_dir=tmp_path): constructor validation, base/failure/decay interval through the sidecar, cross-instance interval sharing, two instances serialize acquire() (the Cross-process rate-limit coordination (file-lock or shared state) #8 criterion), corrupt-sidecar recovery, atomic write (no leftover .tmp)
• Verified the env flag selects the shared limiter (RESEARCH_MCP_S2_SHARED_RATELIMIT=1 → SharedAdaptiveRateLimiter, unset → AdaptiveRateLimiter) under a temp HOME