feat(base-tar): add TarInputStream with GNU/PAX compatibility, plus hardlink and sparse file support

[deer]

base-tar previously had a TarOutputStream but no way to read tar archives back, and the writer had a latent bug where names over 100 characters were truncated at an arbitrary UTF-16 offset instead of a valid path boundary, risking silent corruption of long or multi-byte paths.

This adds TarInputStream, a full POSIX ustar reader with the extensions needed to read real-world archives: PAX extended headers (x/g typeflags, including global-header cancel-on-empty semantics), GNU long-name/long-link extensions (L/K), and the GNU base-256 fallback for numeric fields too large for their fixed-width octal encoding (e.g. sizes at or above 8GB). TarHeader now measures and splits names in UTF-8 bytes rather than UTF-16 characters, fixing the original truncation bug, and falls back to a PAX path record when no valid ustar prefix/name split exists.

Two entry types that were previously unwritable are now supported end to end: hard links (createHardlinkHeader, typeflag 1) and GNU old-style sparse files (createSparseHeader, typeflag S), the latter including multi-block sparse maps for entries with more than four stored chunks. TarOutputStream's PAX writer also now emits size/uid/gid records alongside path/linkpath whenever a PAX header is already being written and one of those fields would otherwise only be recoverable via base-256, for better compatibility with PAX-only readers.

Getting sparse round-trips correct with real tar took two fixes beyond the initial implementation: a missing trailing zero-length terminator entry that GNU tar relies on to truncate extracted files out to their full logical size, and a discovery that GNU tar's own extractor can mishandle sparse chunks at arbitrary byte offsets it did not write itself, so chunk offsets should stay aligned to a filesystem block boundary (as any real sparse-file detector like SEEK_HOLE/SEEK_DATA would naturally produce); this is documented on TarHeader.SparseChunk.