Add S3 VFS backend: VfsBackend impl for S3-compatible storage

- S3Vfs with all 15 VfsBackend methods via rusty-s3 + ureq
- S3VfsFile for buffered writes + ranged reads
- AWS Signature V4 pre-signed URLs (rusty-s3)
- ListObjectsV2 for directory listing (prefix + delimiter)
- Path-style URL mapping (/path to bucket/key)

Cargo.lock

@@ -241,7 +241,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "5ec2f1fc3ec205783a5da9a7e6c1509cc69dedf09a1949e412c1e18469326d00"
 dependencies = [
  "aws-lc-sys",
- "untrusted",
+ "untrusted 0.7.1",
  "zeroize",
 ]
 
@@ -2339,6 +2339,29 @@ dependencies = [
  "hybrid-array",
 ]
 
+[[package]]
+name = "instant-xml"
+version = "0.7.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8863a17b9487acadbfc6a54f1215b67695dcc56d760c69cc08a16ad5e8fd5d0e"
+dependencies = [
+ "instant-xml-macros",
+ "thiserror 2.0.18",
+ "xmlparser",
+]
+
+[[package]]
+name = "instant-xml-macros"
+version = "0.7.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "44127a3a387c070ef0656a6ce53dd0e616cf8d6cf5b159aa478cfd49e1c166e0"
+dependencies = [
+ "heck",
+ "proc-macro2",
+ "quote",
+ "syn 2.0.117",
+]
+
 [[package]]
 name = "internal-russh-forked-ssh-key"
 version = "0.6.9+upstream-0.6.7"
@@ -2677,6 +2700,7 @@ dependencies = [
  "russh",
  "russh-keys",
  "russh-sftp",
+ "rusty-s3",
  "serde",
  "serde_json",
  "sevenz-rust",
@@ -2691,6 +2715,8 @@ dependencies = [
  "tokio-util",
  "toml",
  "unrar",
+ "ureq",
+ "url",
  "uuid",
  "x25519-dalek",
  "xz2",
@@ -4040,6 +4066,20 @@ dependencies = [
  "subtle",
 ]
 
+[[package]]
+name = "ring"
+version = "0.17.14"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a4689e6c2294d81e88dc6261c768b63bc4fcdb852be6d1352498b114f61383b7"
+dependencies = [
+ "cc",
+ "cfg-if",
+ "getrandom 0.2.17",
+ "libc",
+ "untrusted 0.9.0",
+ "windows-sys 0.52.0",
+]
+
 [[package]]
 name = "rocksdb"
 version = "0.24.0"
@@ -4348,12 +4388,66 @@ dependencies = [
  "windows-sys 0.61.2",
 ]
 
+[[package]]
+name = "rustls"
+version = "0.23.40"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ef86cd5876211988985292b91c96a8f2d298df24e75989a43a3c73f2d4d8168b"
+dependencies = [
+ "log",
+ "once_cell",
+ "ring",
+ "rustls-pki-types",
+ "rustls-webpki",
+ "subtle",
+ "zeroize",
+]
+
+[[package]]
+name = "rustls-pki-types"
+version = "1.14.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "30a7197ae7eb376e574fe940d068c30fe0462554a3ddbe4eca7838e049c937a9"
+dependencies = [
+ "zeroize",
+]
+
+[[package]]
+name = "rustls-webpki"
+version = "0.103.13"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "61c429a8649f110dddef65e2a5ad240f747e85f7758a6bccc7e5777bd33f756e"
+dependencies = [
+ "ring",
+ "rustls-pki-types",
+ "untrusted 0.9.0",
+]
+
 [[package]]
 name = "rustversion"
 version = "1.0.22"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b39cdef0fa800fc44525c84ccb54a029961a8215f9619753635a9c0d2538d46d"
 
+[[package]]
+name = "rusty-s3"
+version = "0.10.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "20f0d23aa8ac3b44d4cfb1e4b3611e6f3776debfb3f7701c4ea9f2252a701403"
+dependencies = [
+ "base64",
+ "hmac 0.13.0",
+ "instant-xml",
+ "jiff",
+ "md-5",
+ "percent-encoding",
+ "serde",
+ "serde_json",
+ "sha2 0.11.0",
+ "url",
+ "zeroize",
+]
+
 [[package]]
 name = "ryu"
 version = "1.0.23"
@@ -5372,6 +5466,28 @@ version = "0.7.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "a156c684c91ea7d62626509bce3cb4e1d9ed5c4d978f7b4352658f96a4c26b4a"
 
+[[package]]
+name = "untrusted"
+version = "0.9.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8ecb6da28b8a351d773b68d5825ac39017e680750f980f3a1a85cd8dd28a47c1"
+
+[[package]]
+name = "ureq"
+version = "2.12.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "02d1a66277ed75f640d608235660df48c8e3c19f3b4edb6a263315626cc3c01d"
+dependencies = [
+ "base64",
+ "flate2",
+ "log",
+ "once_cell",
+ "rustls",
+ "rustls-pki-types",
+ "url",
+ "webpki-roots 0.26.11",
+]
+
 [[package]]
 name = "url"
 version = "2.5.8"
@@ -5661,6 +5777,24 @@ dependencies = [
  "wasm-bindgen",
 ]
 
+[[package]]
+name = "webpki-roots"
+version = "0.26.11"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "521bc38abb08001b01866da9f51eb7c5d647a19260e00054a8c7fd5f9e57f7a9"
+dependencies = [
+ "webpki-roots 1.0.8",
+]
+
+[[package]]
+name = "webpki-roots"
+version = "1.0.8"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bf85cb06032201fa7c6f829d7db5a7e5aa45bcc0655327713065f6f0576731bf"
+dependencies = [
+ "rustls-pki-types",
+]
+
 [[package]]
 name = "which"
 version = "7.0.3"
@@ -6182,6 +6316,12 @@ dependencies = [
  "xml",
 ]
 
+[[package]]
+name = "xmlparser"
+version = "0.13.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "66fee0b777b0f5ac1c69bb06d361268faafa61cd4682ae064a171c16c433e9e4"
+
 [[package]]
 name = "xmltree"
 version = "0.12.0"