feat: read tar and tar.gz archives as one workbook (#69)

[vaceslav]

What and why

Closes #69. Part 2 of #76 (more archive formats), after gzip (#80). TabularFile.Open and ArchiveCursor now read a .tar and a .tar.gz / .tgz as one workbook, exactly as a zip: the sheets of every csv, xlsx and ods file inside, in path order, each with its entry's path as Source.

Spec: docs/superpowers/specs/2026-10-03-tar-design.md. Plan: docs/superpowers/plans/2026-10-03-tar.md.

• Public API: only TabularFormat.Tar (for .tar and .tar.gz alike). ArchiveCursor(stream, …) tells zip, tar and tar.gz apart by their bytes; a tar header is known by its magic and checksum, raw or as the first decompressed block of a gzip.
• ArchiveCursor refactored over an internal ArchiveContainer — ZipContainer (today's code, moved), TarContainer, TarGzContainer. Its judging, skipping, bounds and sheet logic is shared; 7z (Read 7z archives #75) will plug in the same way. Zip behaviour is unchanged: every existing test passes untouched.
• .tar: indexed in one pass over the headers (TarReader skips data by seeking); entries are read in place through a seekable window, workbooks included — nothing is copied. The end-of-archive zero block is checked (a seekable TarReader does not), so a tar cut at an entry boundary is refused.
• .tar.gz: one decompression pass lists the sheets; reading a sheet decompresses again up to it, continuing forward when the sheet lies further on. Memory stays flat. A pass that reaches the end reads the gzip stream to its end, so the whole-file CRC is checked.
• Entries: directories, links, devices, hidden paths and macOS ._ files are left out; .gz entries skip as Compressed, a zip or tar inside as NestedArchive. A tar holding a GNU sparse file is refused as format.unsupported — TarReader throws at the sparse header, so nothing after it can be reached.
• TarReader quirk worked around: on .NET 8 and 10 it drops the ustar prefix of a PAX entry that has no path record — which is how bsdtar (the tar of macOS and Windows) writes every path that fits the ustar fields, so a path over 100 characters lost its folders. The prefix is joined from the raw header.
• net8: TarEntry.DataOffset is .NET 9+; the fallback sits in one helper under #if and goes when net8 is dropped at the .NET 11 release.
• Cut or damaged archives raise only format.truncated / format.corrupt; bounds as for zip (MaxEntries, MaxUncompressedBytes — for a tar.gz counted as it is decompressed, MaxEmbeddedWorkbookBytes).

Performance (5M-row csv, 572 MB; Release, each in its own process, machine under load):

	csv	zip	zip on main	tar	tar.gz
read	3.10 s	4.19 s	4.17 s	3.02 s	5.76 s
full analysis	13.0 s	13.8 s	14.1 s	12.9 s	15.6 s
peak RSS (read)	54 MB	57 MB	57 MB	55 MB	56 MB

A tar.gz pays two decompressions (listing, then reading). 20 csv files of 50,000 rows in one tar.gz: stored in path order, read 1.46 s / analysis 3.92 s; stored in reverse path order — one restart per sheet, the worst case — 4.33 s / 6.76 s. Row and cell counts equal across csv, zip, tar and tar.gz.

Tests: every tar format (ustar, PAX, GNU), long paths, every entry type, cuts of .tar and .tar.gz, missing end marker, damaged headers, a tar.gz stored out of path order read back and forth, bounds, detection near-misses, a bsdtar-style PAX entry, zip/tar/tar.gz profiling alike, analysis → plan → import; tar in the stream-ownership, cancellation and API-contract suites; a tar / tar.gz fuzz case (20,000 cases run locally, which found the nameless-entry case on net8, fixed). Checked by hand against real archives made by macOS tar (bsdtar).

Checklist

• A test that failed before the change and passes after it (for a fix or a new behaviour).
• dotnet build and dotnet test pass on net8.0 and net10.0 with zero warnings. (2572 tests, rebased on main)
• If the read path changed: measured, and the numbers are in the description. (zip before/after above)
• If an error code was added or changed: ErrorCodes and the guide's error-code table agree. (no new codes)
• Public API changes are described, and breaking ones are called out. (additive: TabularFormat.Tar; a consumer's exhaustive switch over TabularFormat needs the new member)
• No third-party package in src/.