Fix review findings: PAX sizes, agent stdin, PEM parity, sticky writer

- absorb uses the PAX-corrected entry size (large Go/bsdtar archives were
  stored truncated) and refuses GNU sparse entries
- the agent reads the key from fd 0 straight into locked memory, and never
  unlinks a live agent's socket
- the key file PEM parser is a port of Go's pem.Decode
- the STREAM writer fails permanently after a write error; plaintext is
  cleared on reader error paths; AEAD, HMAC and hash states are zeroized
- add/migrate refuse paths inside the vault; publish reports filesystems
  without an atomic no-replace primitive clearly and treats a leftover
  temporary name as a warning; Windows rejects NTFS-special names
- the leak test runs in its own process

Co-Authored-By: Claude <noreply@anthropic.com>

Cargo.lock

@@ -88,6 +88,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d2f6c7dbe95a6ed67ad9f18e57daf93a2f034c524b99fd2b76d18fdfeb6660aa"
 dependencies = [
  "hybrid-array",
+ "zeroize",
 ]
 
 [[package]]
@@ -121,6 +122,7 @@ dependencies = [
  "cfg-if",
  "cipher",
  "cpufeatures",
+ "zeroize",
 ]
 
 [[package]]
@@ -133,6 +135,7 @@ dependencies = [
  "chacha20",
  "cipher",
  "poly1305",
+ "zeroize",
 ]
 
 [[package]]
@@ -277,6 +280,7 @@ dependencies = [
  "block-buffer",
  "crypto-common",
  "ctutils",
+ "zeroize",
 ]
 
 [[package]]