fix(testing): let the exit-code gate see fail() helpers and tuple returns

[TMHSDigital]

Closes #360.

Problem

tests/check_exit_code_readme.py resolved only return N, sys.exit(N) and named constants. Most showcase checks are written as return fail("...", N), and many return tuples, so most codes went unguarded.

Change

• Pass-through helpers: a module function whose every return hands back one of its own parameters (def fail(msg, code): …; return code) is treated as a pass-through. fail(msg, 15) and fail(msg, code=15) resolve to 15.
• Tuple returns: these resolve through their first element: return 9, objs, objs, return fail(…), None, … and return (fail(…, 11),) + none3.
• New tests/test_exit_code_readme.py (4 cases), added to Validate.

Evidence (local, no Blender)

• The issue's probe: shipping_crate.py copied with its README rows 15–18 deleted.
  • Old checker: exit-code README checks passed.
  • New checker: return/exit 15|16|17|18 is not in the … exit table (4 errors).
• Coverage: distinct non-zero codes the gate sees, summed over entry scripts, went from 559 to 2031. The README tables hold about 2,193 rows; the remainder are render-path or argparse codes documented by hand.
• Full repo: still passes, so no undocumented code was hiding. This matches the review's scan.
• test_exit_code_readme.py: 4/4 OK.

🤖 Generated with Claude Code

[TMHSDigital]

CI evidence before merge: all checks pass ( 13 pass ). Blender Smoke on PR head: Blender 5.2.2 LTS / Blender 4.5.14 LTS. Summaries (happy path; falsifiers) per leg: 143 passed, 1 skipped, 0 failed;142 passed, 0 skipped, 0 failed;141 passed, 3 skipped, 0 failed;136 passed, 6 skipped, 0 failed;