builtin: declare :list:member as ArgModeInputOutput (fixes #94)

[jdymitarai]

Root Cause

In builtin/builtin.go, :list:member was declared with argument modes {ast.ArgModeOutput, ast.ArgModeInput}. The static rule analyzer consequently rejected goals where the member argument is already bound or constant (ANALYSIS ERROR: expected X to be a free variable), even though Decide() in builtin/builtin.go and BoundsAnalyzer.feasibleAlternatives() in analysis/boundscheck.go are explicitly implemented to handle both bound and free member arguments.

Fix

Update :list:member's predicate mode in builtin.go to {ast.ArgModeInputOutput, ast.ArgModeInput}. In addition, fix Windows path separator handling in analysis/validation_test.go (path.Join for embed.FS).

Test Verification

Added bounds check test case in analysis/validation_test.go and an end-to-end stratified program evaluation test TestListMemberBound in engine/seminaivebottomup_test.go. Verified all tests in builtin, analysis, and engine pass (go test -count=1 ./...).