docs: reference the official zstd API manual

Verified the streaming impl against
https://facebook.github.io/zstd/doc/api_manual_latest.html (cParameter/
EndDirective values, compressStream2/decompressStream return semantics, buffer
struct layout — all match). Name the e_flush directive and link the manual from
Bindings and compatibility.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: dfa1

docs/compatibility.md

@@ -1,6 +1,7 @@
 # Symbol compatibility
 
-Coverage of the zstd C API by the `io.github.dfa1.zstdffm` bindings.
+Coverage of the zstd C API by the `io.github.dfa1.zstdffm` bindings. Signatures
+and semantics follow the [official manual](https://facebook.github.io/zstd/doc/api_manual_latest.html).
 
 - zstd version: **1.6.0** (vendored `third_party/zstd`)
 - Public symbols exported by `libzstd`: **186**

zstd/src/main/java/io/github/dfa1/zstdffm/Bindings.java

@@ -9,6 +9,9 @@
 
 /// Central registry of bound zstd C functions.
 ///
+/// Signatures and semantics follow the
+/// [official manual](https://facebook.github.io/zstd/doc/api_manual_latest.html).
+///
 /// `size_t` and `unsigned long long` are modelled as
 /// {@link java.lang.foreign.ValueLayout#JAVA_LONG} (LP64); the public API guards
 /// against negative interpretations where zstd uses sentinel values.

zstd/src/main/java/io/github/dfa1/zstdffm/ZstdOutputStream.java

@@ -24,9 +24,11 @@
 /// }
 public final class ZstdOutputStream extends OutputStream {
 
-    // ZSTD_cParameter / ZSTD_EndDirective values from zstd.h
+    // ZSTD_cParameter / ZSTD_EndDirective values from zstd.h — see
+    // https://facebook.github.io/zstd/doc/api_manual_latest.html
     private static final int ZSTD_C_COMPRESSION_LEVEL = 100;
     private static final int ZSTD_E_CONTINUE = 0;
+    private static final int ZSTD_E_FLUSH = 1;
     private static final int ZSTD_E_END = 2;
 
     private final OutputStream out;
@@ -101,7 +103,7 @@ public void flush() throws IOException {
         in.set(inSeg, 0, 0);
         long remainingHint;
         do {
-            remainingHint = drainOutput(1 /* ZSTD_e_flush */);
+            remainingHint = drainOutput(ZSTD_E_FLUSH);
         } while (remainingHint != 0);
         out.flush();
     }