[jeandet]

Adds what a parser needs to read a remote file (S3, HTTP) lazily, fetching each byte at most once. There is no network code: the transport is supplied by the caller.

• range_reader: a concept for "give me these byte ranges", batched so a transport can fetch them concurrently. The batch fills every range or throws.
• local_range_reader: a range_reader over a local file that logs every call. It is used to test and measure what a remote reader would be asked for.
• reserved_region: memory the size of the file, whose pages can't be read until they are written and sealed. Backed by anonymous memory, or by a deleted temp file the OS can page out. Linux, macOS and Windows paths are included; only Linux was compiled locally.
• remote_buffer: the cache. ensure() widens each read to a read-ahead window, merges what is missing and fetches it in one batch. The first fetch also takes the tail of the file. Unfetched bytes raise in check_present() and fault on a raw read, so they are never read as zeros. It is thread-safe and never calls the reader under its lock. A reader can supply blocking_wait to release an outer lock (Python's GIL) while a thread waits for another thread's fetch.

Used by CDFpp for remote CDF reads (branch feat/remote-reads, opened once this PR is merged and the wrap is bumped).

Tests: 38/38. TestRemoteBuffer also passes under ThreadSanitizer. Fork-based probes check that unsealed pages fault.

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