feat(#83): archive VFS backend - read-only FileSystem over one container

[helly25]

Implements the @xff_extras_api VFS seam inside @xff_archive (never the core), spelling member paths through member_path_cc, so the whole expression grammar matches an archive's members at virtual paths.

Each decision is pinned by a test over a real tar the test writes itself:

• Implicit parent directories are SYNTHESIZED. The fixture deliberately stores dir/sub/deep.txt with no entry for dir/ or dir/sub/, exactly as real tar streams do - without synthesis a walk finds nothing to descend into.
• read_only + Source::kArchiveMember on every member - the pair that makes -delete and the exec family REFUSE members instead of silently skipping them.
• Remove returns PermissionDenied, never silent success (which would make -delete look like it worked). Access is never writable and otherwise follows the stored mode bits. ReadLink resolves a symlink member and refuses a regular one. FsType is "archive"; IsCaseSensitive is true, since stored names are bytes.
• A path outside the container, a missing member, and "not a directory" are three distinct errors, not an empty listing.
• The configured separator is used for rendering AND parsing - tested with #, so a path the backend printed is one it can stat back.
• ReadContent returns Unimplemented on purpose: the reader lists members but does not extract data, and an empty string would make -grep / -content silently match nothing. Member extraction is the next slice.

Two things found while writing it, both recorded rather than smoothed over:

• Inside namespace xff::archive, unqualified archive resolves to the namespace, not libarchive's struct archive - the test needs ::archive.
• Reader asymmetry: ListMembers("") rejects empty input as InvalidArgument, but ListMembersOfFile on an empty FILE opens fine and reports zero members (libarchive hits EOF immediately). A test pins today's behaviour; the two entry points should probably agree, which is a reader-level call.

Verified: 14 backend tests plus the reader's own pass under --//xff:xff_archive; //... is 94 green; pre-commit run --all-files clean.

Left for follow-ups: engine-side mounting when --archive selects a container (where --archive-depth and sniff-gating land), and ReadMemberOfFile.

NOTE: not armed - it queues behind #454 and #450 per your ordering. Its TODO.md note is intentionally absent here; it goes in the TODO.md repair PR, since that file needs a structural fix first (my nested bullets were being reflowed by prettier).