feat: receiver-side basis matching with link/copy materialization

The receiver's per-file incremental check now consults the ordered basis-dir
list whenever the destination is not already up to date.  An exact basis match
requires equal size, equal mtime (unless --size-only; --ignore-times disables
basis matching like rsync), and an equal content xxHash64 -- the sender sends
its xxHash for every file whenever basis dirs are configured (not only under
--checksum), so a hard link or local copy is only ever made from byte-identical
content.

On a match:
  - compare-dest: reply STATUS_OK and skip data only when the destination does
    not already hold the file (sparse, rsync parity).  A destination that holds
    a DIFFERENT version falls back to a normal transfer instead of rsync's
    delete, keeping the mirror complete.
  - copy-dest: reply STATUS_OK and hand a synthetic File (bytes read from the
    basis file, basis metadata) to the normal store sink, so the file is
    installed as a real local copy through the existing atomic temp+rename
    engine and honors --existing/--ignore-existing/--update/--backup/
    --delay-updates/--partial-dir unchanged.
  - link-dest: same, but File.basis_link records the basis path and the store
    engine calls the new file_to_disk_secure_link(): an atomic temp hard link +
    rename.  Cross-filesystem/refused links fall back to a byte-identical local
    copy (never a corrupt or partial file); the copy fallback applies metadata,
    while a successful link keeps the basis inode's own attributes so the basis
    file is never mutated.

Basis-materialized files carry File.skip so they are not acknowledged to a
--remove-source-files sender (the sender already saw STATUS_OK and keeps the
source).  The no-match path is byte-for-byte identical to the existing delta /
full-data transfer.

src/server/receiver.c

@@ -214,7 +214,7 @@ static bool receiver_save_file(File* file, void* context_pointer) {
     result = file_save_to_disk_full(context->config->receive_root_directory, file, context->config);
   }
   if (result != FILE_SAVE_ERROR && context->config->remove_source_files && !file->is_dir &&
-      !receiver_outcomes_append(&context->outcomes, (unsigned char)result)) {
+      !file->skip && !receiver_outcomes_append(&context->outcomes, (unsigned char)result)) {
     file_destroy(file);
     return false;
   }