feat(umbp): chunked DRAM MR registration for NICs with limited max_mr_size

[maning00]

Summary

• Split DRAM tier ibv_reg_mr registration into chunks to prevent abort when
  the NIC's max_mr_size is smaller than DRAMTier capacity
• Auto-detect device max_mr_size at runtime via ibv_device_attr; also
  supports UMBP_MAX_MR_CHUNK_SIZE env var and max_mr_chunk_size config
  field for manual override
• Hard-cap IONIC (Pensando) NICs to 4 GB since they report incorrect
  max_mr_size
• Fix Heartbeat() silently discarding tier_capacities updates

Changes

IOEngine / RDMA layer:

• Add Backend::GetMaxMemoryRegionSize() virtual method; RdmaBackend
  queries min(max_mr_size) across all devices with a 4 GB cap for Pensando
• IOEngine exposes GetMaxMemoryRegionSize() delegating to backends

PoolClient auto-chunking:

• Init() detects chunk size, splits DRAM buffer registration into
  per-chunk export_dram_mems_ entries; normalizes to SIZE_MAX (no
  chunking, zero overhead) when effective_chunk >= max(buffer_size)
• RegisterMemory() splits into chunks; RegisteredRegion gains a
  group_base field so DeregisterMemory() can batch-remove all chunks
  belonging to one original registration call

DRAMTier allocator:

• Add SetAlignmentBoundary() to guarantee Allocate() never returns a
  slot that crosses a chunk boundary

Location ID mapping:

• LocalStorageManager::BuildTierLocationInfo() changed from static to
  instance method; maps global offset → chunk_index:chunk_offset when
  chunking is active
• UMBPClient::MaybePublishLocal() adapted accordingly

Config & bindings:

• UMBPDistributedConfig / PoolClientConfig gain max_mr_chunk_size
• pybind exposes the new field
• UMBPClient constructor reads UMBP_MAX_MR_CHUNK_SIZE env var

Bug fix:

• ClientRegistry::Heartbeat() had (void)tier_capacities — capacity
  updates from clients were silently dropped. Now writes them into
  ClientRecord.

Test plan

• All 15 UMBP unit tests pass (including previously failing
  HeartbeatUpdatesCapacities)
• Python binding verified: max_mr_chunk_size is readable/writable
• BUILD_UMBP=1 BUILD_TESTS=1 pip install . builds cleanly
• Integration test with UMBP_MAX_MR_CHUNK_SIZE=1048576 for
  distributed Put/Get across chunk boundaries
• On-device validation with IONIC NIC