refactor: split close.c into reader/writer

Separate writer finalization from resource cleanup in close.c to
enable a future reader-only library target (aaruformatread).

close_write.c contains all write_* helpers and aaruf_finalize_write().
close.c retains aaruf_close() with cleanup only, dispatching writer
finalization via a function pointer set by aaruf_create()/aaruf_open().

Zero behavioral change — same tests pass, same error semantics.

include/aaruformat/context.h

@@ -296,6 +296,7 @@ typedef struct aaruformat_context
     bool        rewinded;            ///< True if stream has been rewound after open (write path).
     bool        writing_long;        ///< True if writing long sectors
     bool        block_zero_written;  ///< True if block zero has been written (writing path).
+    int32_t (*finalize_write)(struct aaruformat_context *ctx);  ///< Writer finalization hook (NULL for reader).
 
     /* Options */
     uint32_t lzma_dict_size;       ///< LZMA dictionary size (writing path).

include/internal.h

@@ -63,6 +63,7 @@ bool    set_ddt_tape(aaruformat_context *ctx, uint64_t sector_address, uint64_t
 aaru_options parse_options(const char *options, bool *table_shift_found);
 uint64_t     get_filetime_uint64();
 int32_t      aaruf_close_current_block(aaruformat_context *ctx);
+int32_t      aaruf_finalize_write(aaruformat_context *ctx);
 int          compare_extents(const void *a, const void *b);
 void         generate_random_bytes(uint8_t *buffer, size_t length);