v0.2.9 — Windows SWMR fix

Bug Fixes

• Windows: SwmrFileWriter::start_swmr no longer attempts to downgrade the writer's exclusive lock to shared. Windows' LockFileEx is a mandatory range lock, and a same-handle unlock-then-shared-relock left subsequent WriteFile calls failing with ERROR_LOCK_VIOLATION (33). The writer now releases its lock entirely when SWMR mode starts, matching the HDF5 C library — which also relies on the SWMR file-format sentinel rather than OS locks during streaming.

  Trade-off: a second writer attaching to a streaming SWMR file is no longer blocked by an OS lock; the SWMR protocol's single-writer guarantee is the caller's responsibility (already documented).

Internal

• CI: cargo fmt --all -- --check now passes (0.2.8 introduced unformatted lines).
• The downgrade_to_shared helper is removed (no production callers remain).