Add opt-in Snappy page compression to reduce database size

Introduce a `Compression` option on DB/Options that enables transparent
Snappy compression of leaf and branch page data. Compression happens at
node spill time—before page allocation—so fewer pages are allocated for
compressible data. Decompression is transparent on read via a
per-transaction cache using sync.Map for concurrent reader safety.

Key changes:
- New orthogonal CompressedPageFlag (0x20) on page headers; type checks
  changed from == to bitwise & so the flag coexists with page types
- CompressInodes serializes and compresses node data, only used when it
  reduces the page count
- Split threshold increased 4x when compression is enabled so nodes
  accumulate enough data for meaningful compression
- DecompressPage preserves on-disk overflow for correct freelist accounting
- Adds github.com/golang/snappy dependency

This was largely written by Claude Opus.

Signed-off-by: Thomas Jungblut <tjungblu@redhat.com>

Author: tjungblu

bucket.go

@@ -884,6 +884,11 @@ func (b *Bucket) node(pgId common.Pgid, parent *node) *node {
 		})
 	}
 
+	// If the page is compressed, decompress it before reading.
+	if p.IsCompressed() {
+		p = b.tx.decompressedPage(p)
+	}
+
 	// Read the page into the node and cache it.
 	n.read(p)
 	b.nodes[pgId] = n
@@ -945,7 +950,14 @@ func (b *Bucket) pageNode(id common.Pgid) (*common.Page, *node) {
 	}
 
 	// Finally lookup the page from the transaction if no node is materialized.
-	return b.tx.page(id), nil
+	p := b.tx.page(id)
+
+	// If the page is compressed, decompress it transparently.
+	if p.IsCompressed() {
+		p = b.tx.decompressedPage(p)
+	}
+
+	return p, nil
 }
 
 // BucketStats records statistics about resources used by a bucket.