feat(ecstore): closed-form range seek for single-part v2 encrypted objects (#6601)

Single-part encrypted objects in the legacy format could not serve range
reads without decrypting from byte 0: v1 frames are emitted per upstream
read, so no closed-form plaintext-to-ciphertext mapping exists. The v2
layout fixed the frame length (8218 ciphertext bytes per 8 KiB plaintext
frame), making the mapping closed-form.

Consume it:
- Single-part PUTs that encrypt locally under the v2 write switch stamp
  the frame-layout marker, valued with the object's data_dir token -
  ciphertext passthrough, data movement and copies mint a new data_dir
  or strip the marker, so a re-homed marker disqualifies itself.
- The encrypted read plan seeks marked, uncompressed single-part objects
  to frame_index * 8218 and decrypts from that frame: the frame index
  rides the plan's sequence-number slot into DecryptReader::new_at_block,
  whose nonce and AAD bind absolute indices. New metric path label
  frame_seek.
- A lying marker fails closed: v2 authentication rejects bytes at a fake
  frame boundary; plaintext is never served from the wrong offset.

Compressed objects and multipart sub-part seeks keep the conservative
paths (follow-up work); reading needs no switch - seekability follows
the marker.

crates/e2e_test/src/kms/mod.rs

@@ -48,6 +48,9 @@ mod encryption_metadata_test;
 #[cfg(test)]
 mod copy_object_self_copy_sse_test;
 
+#[cfg(test)]
+mod encrypted_range_get_test;
+
 #[cfg(test)]
 mod copy_object_version_restore_sse_test;