Fix issue #50: SASLib.open hangs for large files

[tk3369]

Problem

SASLib.open and SASLib.read both fail on large SAS7BDAT files (e.g. a 2 GB file with 4,000+ columns). Fixes #50.

Confirmed behaviour on master against the Census AHS 2013 public-use file (2.0 GB, 70,044 rows x 4,041 columns):

• open() completes but is ~2x slower than necessary (1.476s vs 0.785s)
• read() crashes with BoundsError: attempt to access 1355-element Vector{UInt8} at index [1356] in populate_column_indices

Root cause

Two bugs in the metadata-reading path:

1. read_file_metadata ignored _process_page_meta's return value

_process_page_meta returns true when it has finished reading metadata (i.e. it has reached the first data page). The loop was discarding that signal and continuing to scan every page in the file — for a wide file this means reading through thousands of data pages unnecessarily.

2. _process_columnattributes_subheader used replace semantics instead of accumulate

The function called empty! before append!, so for files whose column attributes span multiple metadata pages (wide files with 4,000+ columns), only the last page's chunk of attributes survived. Meanwhile column_symbols accumulated all column names across pages, producing a length mismatch that caused the BoundsError in populate_column_indices.

A secondary issue: the guard condition used length(handler.column_types) where length is also a parameter name, causing a MethodError: objects of type Int64 are not callable. Fixed by qualifying as Base.length.

Fix

• read_file_metadata: break out of the loop when _process_page_meta returns true
• _process_columnattributes_subheader: switch to append!-only (accumulate across pages); add a Base.length guard to skip re-processing once all column attributes are loaded

Testing

• All 237 existing tests pass
• Manual regression against the Census AHS 2013 public-use file (2.0 GB, 70,044 rows x 4,041 columns):

	master	this PR
open()	1.476s	0.785s
read(1000)	BoundsError	0.839s
read() full file	BoundsError	12.04s

A new manual regression script test/test_large_sas.jl is included with download instructions for the test file. It is intentionally excluded from the standard Pkg.test() suite since the file must be downloaded separately.