fix(receiver): non-blocking receiver opens, inplace type gate, dry-run/B4/B5/B6

Address confirmed receiver security findings B1-B6:

B1 (HIGH): add O_NONBLOCK to the three receiver read-opens that opened an
existing destination/basis entry before the S_ISREG gate
(incremental_check_open_destination, basis_open_regular, hardlink_read_source)
so a client-planted FIFO can no longer block the receive thread forever while
the post-open type gate still rejects it.

B2 (HIGH/MED): --inplace now fstatat(AT_SYMLINK_NOFOLLOW)-probes the target and
refuses any existing non-regular entry, opens with O_NONBLOCK, and re-checks
S_ISREG on the opened fd.  This stops a FIFO from hanging the open and stops a
char/block device from being written directly (bypassing --write-devices).

B3 (MED): under --dry-run the incremental quick-skip no longer reads/hashes the
destination file for --checksum/--delta; it decides from metadata only and
reports would-transfer when the comparison is inconclusive, closing the
read-only-module content-hash oracle.

B4 (LOW): xattr_name_appliable() now gates the two system.posix_acl_* names on
preserve_acls (--acls), not the derived use_xattrs (--xattrs OR --acls).  The
receiver drops (never applies) ACL entries when -A was not negotiated while
keeping user.* working for -X.

B5 (INFO): receive_manifest_section() charges a per-entry overhead against
MAX_MANIFEST_BYTES and the aggregate entry count across all three sections is
capped at MAX_MANIFEST_ENTRIES.

B6 (MED): data_charge_session() reserves decompressed/chunk-copy bytes against
the owning ProtocolSession (MAX_CONNECTION_MEMORY) and records them on the Data
so data_destroy() releases them via the Data.owner path.  Applied to the
whole-file/append/delta decompression sites and chunk_deserialize() per-file
copies; a missing session owner degrades to the previous uncharged behavior.

Tests: FIFO destination/basis non-hang (with alarm), --inplace FIFO/device
refusal, dry-run no-read oracle test plus updated metadata-only dry-run tests,
ACL-without--acls drop, manifest total-entry cap, and chunk session charging.

src/shared/xattr.h

@@ -59,9 +59,11 @@ void xattr_list_free(FileXattrList* list);
 bool xattr_list_append(FileXattrList* list, const char* name, const void* value, size_t value_len);
 
 /* True when `name` is a well-formed xattr name AND belongs to a namespace this
- * build is authorized to apply (user.* or the two POSIX ACL xattrs).  Used for
- * both capture and receiver-side validation. */
-bool xattr_name_appliable(const char* name);
+ * build is authorized to apply.  `user.*` is always accepted for -X; the two
+ * POSIX ACL xattrs are accepted only when `preserve_acls` (--acls/-A) is set, so
+ * a plain -X run can never carry or apply an ACL the receiver did not ask for.
+ * Used for both capture and receiver-side validation. */
+bool xattr_name_appliable(const char* name, bool preserve_acls);
 
 /* Sender: read the whitelisted xattrs of `path` into a new list.  Returns NULL
  * when the path has no appliable xattrs (or the filesystem has no xattr
@@ -70,9 +72,12 @@ FileXattrList* xattr_capture_path(const char* path);
 
 /* Wire: bounded serialization.  xattr_send returns false on write failure; an
  * empty/NULL list transmits a zero-count block.  xattr_receive returns NULL and
- * sets *ok = 0 on any malformed / oversized / non-whitelisted entry. */
+ * sets *ok = 0 on any malformed / oversized / non-whitelisted entry.  When
+ * `preserve_acls` is false, any POSIX ACL entries are consumed and DROPPED (so
+ * a -X transfer still succeeds and never applies an ACL it did not negotiate);
+ * a genuinely disallowed namespace is still rejected. */
 bool xattr_send(int fd, const FileXattrList* list);
-FileXattrList* xattr_receive(int fd, int* ok);
+FileXattrList* xattr_receive(int fd, int* ok, bool preserve_acls);
 
 /* Receiver: apply every entry fd-relative (fsetxattr) to the just-written file
  * descriptor.  A per-attribute failure (e.g. ACL set refused for non-root on a