feat: add shared read/write buffer for sequential files (COB_FILE_SEQ_BUFFER_SIZE)

[yutaro-sakamoto]

Summary

• Introduce a new environment variable COB_FILE_SEQ_BUFFER_SIZE that enables buffering for both
  read and write operations on SEQUENTIAL and LINE SEQUENTIAL files
• The old COB_FILE_SEQ_WRITE_BUFFER_SIZE (write-only) is deprecated and used as a fallback when
  the new variable is not set
• Significantly improves read performance by eliminating 1-byte-at-a-time FileChannel.read()
  calls

Changes

• FileIO.java: Replace separate read/write buffers with a unified buffer using a 3-state
  mode machine (BUFFER_MODE_NONE / READ / WRITE). Mode transitions handle flushing
  (WRITE→READ) and FileChannel position adjustment (READ→WRITE). Also fixes getc() to return
  unsigned byte values (& 0xFF).
• CobolUtil.java: Rename fileSeqWriteBufferSize → fileSeqBufferSize. Read
  COB_FILE_SEQ_BUFFER_SIZE first, falling back to COB_FILE_SEQ_WRITE_BUFFER_SIZE. Add
  NumberFormatException handling and use CobolUtil.getEnv() for consistency.
• CobolFile.java: Expand buffer preparation in open_() from OUTPUT/EXTEND only to all open
  modes (INPUT, OUTPUT, I-O, EXTEND).
• doc/environment_variables.md / doc/environment_variables_JP.md: Add
  COB_FILE_SEQ_BUFFER_SIZE documentation and mark COB_FILE_SEQ_WRITE_BUFFER_SIZE as deprecated.
• tests/misc.src/seq-buffer.at: Add 10 integration tests covering sequential read, line
  sequential read, buffer size 1, buffer disabled, I-O rewrite, many records, deprecated fallback,
  mode transitions, and short records.