test: add 14 Oils-inspired spec test files (244 tests)

[chaliy]

Summary

• Add 14 new spec test files inspired by the Oils (oilshell) test suite covering previously untested bash behaviors
• 244 new tests total: 108 passing, 136 skipped with TODO markers identifying unimplemented features
• Each file links to the original Oils test for reference

High-value test coverage (7 files, 139 tests):

File	Tests	Pass	Skip	Gaps found
word-split.test.sh	39	3	36	IFS splitting, $@/$*, word elision
quote.test.sh	22	11	11	$'', $"", adjacent single quotes
parse-errors.test.sh	18	7	11	Incomplete constructs, unterminated quotes
shell-grammar.test.sh	22	22	0	All pass
empty-bodies.test.sh	8	3	5	Empty loop/function bodies not rejected
exit-status.test.sh	17	8	9	Exit code not truncated to 8-bit
subshell.test.sh	13	9	4	Function/cd/trap/positional leak

Medium-value test coverage (7 files, 105 tests):

File	Tests	Pass	Skip	Gaps found
nameref.test.sh	14	0	14	local -n/typeset -n not implemented
var-op-test.test.sh	19	5	14	${@-val}, ${arr[@]+val}, backslash escapes
heredoc-edge.test.sh	16	10	6	Multiple heredocs in conditions, pipe ordering
unicode.test.sh	17	11	6	$'\u...', printf \u, ${#x} byte vs char
alias.test.sh	15	0	15	Alias expansion not implemented
temp-binding.test.sh	10	10	0	All pass
arith-dynamic.test.sh	14	9	5	Recursive var deref, array access in $((...))

Test plan

• cargo test --test spec_tests passes (all 13 suites green)
• cargo test --test spec_tests -- bash_comparison_tests passes
• Rebased on latest main
• No conflicts with recent main changes