Add portable Windows, macOS, and Android backend support

[amachado-pie]

This PR refactors dqlite's Linux-specific paths into selectable portable backends so the project can build and validate on Windows, macOS, and Android while retaining optimized Linux backend options.

Summary

• Add Autotools switches for VFS shared-memory and Raft I/O backend selection:
  • --with-vfs-shm=auto|linux-remap|portable
  • --with-raft-io=auto|portable|linux-kaio
• Split the Raft writer into:
  • a default portable LibUV backend using uv_fs_write/uv_fs_close
  • an optional Linux-only KAIO backend
• Add a portable heap-backed SQLite VFS shared-memory provider while keeping the Linux remap provider as the optimized Linux path.
• Replace server synchronization and lifecycle primitives with LibUV equivalents.
• Add Windows portability wrappers for sockets, address parsing, filesystem operations, locking, and test addresses.
• Add Android portability fixes, including aligned Raft allocation without relying on aligned_alloc().
• Add macOS/Darwin portability fixes for Apple SQLite hooks, TCP reset behavior, resolver interposition limits, filesystem helper APIs, and BSD qsort_r.
• Gate or adapt platform-specific tests for Linux, Windows, macOS, and Android assumptions.
• Add documentation for:
  • cross-platform porting and validation
  • Windows MinGW/Wine builds
  • Android NDK cross-builds
  • native macOS/Homebrew builds

Validation performed

• Native Linux make check: 7/7 test programs passing.
• Linux optional KAIO backend make check: 7/7 test programs passing.
• Windows/MinGW build under Wine:
  • integration-test.exe non-stress: 127/127 passing, 1 skipped.
  • integration-test.exe stress/read_write: 45/45 passing, 3 skipped.
  • unit-test.exe: 75/75 passing, 3 skipped.
  • raft-core-unit-test.exe: 135/135 passing.
  • raft-core-integration-test.exe: 191/191 passing, 3 skipped.
  • raft-uv-unit-test.exe: 20/20 passing, 61 skipped.
  • raft-uv-integration-test.exe: 213/213 passing, 25 skipped.
  • raft-core-fuzzy-test.exe: 57/57 passing.
• Native macOS make check: 7/7 test programs passing.
  • integration-test: 170/170 passing, 4 skipped.
  • raft-uv-unit-test: 24/24 passing, 57 skipped.
  • raft-uv-integration-test: 220/220 passing, 22 skipped.
  • raft-core-fuzzy-test: 57/57 passing.
• Android NDK cross-build with NDK 27.2.12479018, API 24:
  • arm64-v8a: libdqlite.la builds successfully.
  • armeabi-v7a: libdqlite.la builds successfully.
• git diff --check passes.

Benchmark notes

The portable stress/read_write benchmark was recorded for Linux portable, Windows/Wine portable, and native macOS portable builds using seed 0x5eed1234, one warmup iteration, and 10 measured iterations.

Scenario	Linux portable wall	Windows/Wine portable wall	Windows delta	macOS portable wall	macOS delta
writers=0 readers=16 databases=1	6.71949480s	25.02570840s	+272.43%	8.73973800s	+30.07%
writers=1 readers=4 databases=1	8.03535353s	8.93448470s	+11.19%	2.56766500s	-68.05%
writers=4 readers=0 databases=1	28.66580590s	9.71652050s	-66.10%	8.39178000s	-70.73%

These numbers are intended as cross-platform validation data, not direct hardware or native Windows performance rankings.

Known follow-up

• Add CI jobs for Linux portable/KAIO, Windows/Wine, macOS, and Android cross-build checks.
• Add Android emulator/device smoke tests for storage, VFS, and TCP loopback.
• Add native Windows runner coverage if suitable CI capacity is available.
• Continue optimizing the portable heap-backed VFS provider for concurrent read workloads.