Add comprehensive regression test script (closes #83)

[tk3369]

Summary

• Adds test/regression.jl — a standalone script that computes SHA256 digests of all 34 SAS7BDAT test files and supports before/after comparison to catch data-reading regressions.

Usage

# Before making changes (on parent commit):
julia test/regression.jl save regression_baseline.txt

# After making changes:
julia test/regression.jl compare regression_baseline.txt

Exit code 0 = all match, 1 = differences found (CI-friendly).

How it works

• Covers all 6 test data directories (34 files); skips zero_variables.sas7bdat and gracefully handles any file that fails to read
• SHA256 is computed over: row/column counts, column names, and each column's raw data bytes (type-dispatched: Float64 → raw IEEE-754 bytes, strings → UTF-8 + null separators, dates/datetimes → Int64 epoch values)
• Pkg.activate only fires when run as the entry-point script

How I tested this

Save mode — ran julia test/regression.jl save /tmp/clean_baseline.txt and confirmed all 34 files were processed and the digest file written correctly.

Compare mode (no change) — ran julia test/regression.jl compare /tmp/clean_baseline.txt immediately after; confirmed output: 34 / 34 files match. All files match. No regressions detected.

Compare mode (injected regression) — temporarily patched src/SASLib.jl to add 999.0 to the first float value of every numeric column when reading data_AHS2013/homimp.sas7bdat, then re-ran compare. Output:

33 / 34 files match

DIFFERENCES (1 files):
  DIFF  data_AHS2013/homimp.sas7bdat
        baseline: 408b3e1493f8319edc26f3720a70e3119f7733de9bdf66ed3cee8785417c4fdf
        current:  9aa5acb78c18e503d775be5b65e49d6fb6d7591adb9b871a5e38e945b7588a99

The script correctly detected the single corrupted file and reported its before/after digests.

Related

• Closes Comprehensive regression test #83
• Known limitation tracked in regression.jl: fallback update_column! uses repr() which is not stable across Julia versions #85: the repr()-based fallback for unrecognized column types is not Julia-version-stable (low severity, unreachable with default readsas args)

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