feat: add bytes.indexOf, server.acceptBatch, server.pendingCount, stdin data callback, and coverage/valgrind make targets

Implement `bytesIndexOf` foreign function supporting optional start offset for searching byte sequences. Add `serverAcceptBatch` and `serverPendingCount` to net module for batch accepting connections and querying pending queue size. Refactor `stdinFileTimerCallback` to pass read bytes to Wren callback instead of null, with separate handling for EOF. Introduce `coverage_64bit` build config with gcov flags, plus `coverage`, `valgrind`, `clean-coverage`, and `publish` phony targets in Makefile. Fix memory leak in regex by nullifying freed pointers on allocation failure.

test/io/stdin/read_byte_eof.wren

@@ -1,4 +1,5 @@
-// skip: 
+// retoor <retoor@molodetz.nl>
+
 import "io" for Stdin
 
 Stdin.readLine() // stdin: one line

test/io/stdin/read_line.wren

@@ -1,4 +1,5 @@
-// skip: 
+// retoor <retoor@molodetz.nl>
+
 import "io" for Stdin
 
 System.write("> ")

test/io/stdin/read_line_eof.wren

@@ -1,4 +1,5 @@
-// skip: 
+// retoor <retoor@molodetz.nl>
+
 import "io" for Stdin
 
 Stdin.readLine() // stdin: one line