[lemire]

This supersedes #30 by @vsarunas. #30 conflicts with master and comes from a fork that doesn't allow maintainer edits, so it couldn't be updated in place.

From #30

The four commits are cherry-picked onto master unchanged, and @vsarunas remains their author:

• ptr and frozenBacking become let, which removes two dynamic exclusivity checks from every deinit. Census1881 random access gets about 20% faster; the benchmark is in perf: make bitmap storage immutable, test from Swift 5.6 #30.
• RoaringBitmap's decoder throws DecodingError on corrupt input instead of crashing.
• CI runs in swift:5.6-focal, 5.10, 6.2 and latest containers, replacing fwal/setup-swift, which doesn't support Ubuntu 26.04. swift-tools-version goes to 5.6, and actions/checkout to v7.

The only conflict was tests.yml. #30's container matrix replaces the stopgap in f594ff9.

Added here

• Both decoders now validate. CRoaring's safe deserializers only bound their reads. A payload with a well-formed header over bad containers still gives a bitmap, and CRoaring's header warns that using it may crash. Both init(from:) now call internal_validate and throw DecodingError.dataCorrupted with CRoaring's reason. New tests swap the last two values of an encoded array container. Before this change, that input decoded without an error.
• Clearer errors. The 32-bit decoder reports invalid base64 and an unreadable payload separately, like the 64-bit one.
• README badge. It points at the renamed workflow (tests.yml).

Testing

• macOS, Swift 6.2.4: 91 tests pass with swift test, --sanitize=address and --sanitize=undefined.
• Linux containers: swift:5.6-focal and 5.10 run 89 tests (the RawSpan tests need 6.2), and 6.2 and latest (6.4.1) run 91. All pass.