Read a disc folder as an input: dir:// becomes a source

Users keep discs as extracted folders — a DVD VIDEO_TS or a Blu-ray BDMV,
usually a backup that is already decrypted. dir:// could only ever be a
destination, so those folders could be produced and never read back.

Everything above the sector layer wants a UdfFs over a SectorSource, and
every UdfFs read re-reads the ICB off that source at call time, so a
folder has to present itself as sectors. It does: dirimage plans a block
layout over the real files, encodes a UDF 1.02 filesystem for the
metadata, and serves data straight from disk. read_filesystem then parses
it exactly as it parses a disc, so nothing above changes — and the
iso:// arm of input() is now shared rather than duplicated, so dir://
inherits its decrypt gates, title selection and stream pruning.

The encoder is validated by more than its own reader: macOS mounts the
synthesized image and the mounted files compare byte-identical to the
originals. A round-trip through our own parser could not have shown that
— the tag CRC seeds at zero, and a wrong seed would satisfy us and no
real driver.

DVD placement is not free packing: a VTS IFO records where its title
VOBS begins relative to itself, so the VOB has to land exactly there.
Unsatisfiable marks fail loudly rather than misplace the file. 3D folders
are refused for now: the scanner detects SSIF and the planner cannot
alias its extents yet, so accepting them would produce quiet nonsense.

Left for later: metadata capture, HD-DVD, FMTS, encrypted folders.

src/lib.rs

@@ -106,6 +106,7 @@ pub mod consts;
 pub mod css;
 pub mod decrypt;
 pub mod diag;
+pub mod dirimage;
 pub mod disc;
 pub mod drive;
 pub mod dvdnav;
@@ -148,7 +149,8 @@ pub use drive::{Drive, DriveStatus, extract_scsi_context, find_drive};
 // Owns the `Drive` by value; forwards consumer-built key material into
 // `ScanOptions` (the library derives no certs — see `KeySpec`).
 pub use session::{
-    DeviceTarget, DiscSession, KeySourceFactory, KeySpec, ResolvedKeys, resolve_keys_for, scan_iso,
+    DeviceTarget, DiscSession, KeySourceFactory, KeySpec, ResolvedKeys, resolve_keys_for, scan_dir,
+    scan_iso,
 };
 
 // ─── Errors ─────────────────────────────────────────────────────────────────
@@ -228,6 +230,7 @@ pub use decrypt::{AacsKeyMap, DecryptKeys, decrypt_sectors, decrypt_threads, set
 // — not the `pes::Stream` trait re-exported below as `PesStream`. Two
 // different concepts, the same short name; the trait gets the `Pes`
 // prefix at the crate root to keep both addressable.
+pub use dirimage::DirImage;
 pub use disc::{
     AacsState, AudioChannels, AudioStream, Clip, Codec, ColorSpace, ContentFormat, Disc,
     DiscFormat, DiscId, DiscTitle, DriveCredentials, Extent, ExtractOptions, ExtractResult,