fix: restore preamble-only PCH hash to fix shared preamble test

The previous change hashed compile arguments (including the source file
path) into the preamble hash, causing files with identical preambles but
different paths to generate separate PCH files. Revert to hashing only
the preamble text, matching the behavior before this refactor.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 16bit-ykiko

src/server/compiler.cpp

@@ -445,14 +445,13 @@ et::task<bool> Compiler::ensure_pch(Session& session,
         co_return true;
     }
 
-    // Hash both the preamble text AND compile arguments so that different
-    // compilation contexts (e.g. after switchContext) produce different PCHs.
+    // Hash only the preamble text.  This allows files with identical preambles
+    // to share the same PCH file (content-addressed).  Compile flags are NOT
+    // included because preamble content is what determines the PCH output.
+    // TODO: consider hashing flags that affect preprocessing (e.g. -D, -I)
+    // for correctness when files have same preamble text but different flags.
     auto preamble_text = llvm::StringRef(text).substr(0, bound);
     auto preamble_hash = llvm::xxh3_64bits(preamble_text);
-    for(auto& arg: arguments) {
-        preamble_hash ^= llvm::xxh3_64bits(arg);
-    }
-    preamble_hash ^= llvm::xxh3_64bits(directory);
 
     // Deterministic content-addressed PCH path.
     auto pch_path = path::join(workspace.config.cache_dir,