feat: cat -v, sort -m, brace/date/lexer fixes

[chaliy]

Summary

• cat -v/-n/-e/-t: Show non-printable characters (^M for CR, ^[ for ESC, etc.)
• sort -m: Merge pre-sorted files via k-way merge
• Brace space fix: echo { a,b,c } no longer triggers brace expansion or compound command parsing; { and } treated as literal words in argument position
• Date format fix: date +"%Y-%m-%d %H:%M:%S" now works — strip surrounding quotes from format argument
• Lexer word concatenation: Adjacent quoted segments are concatenated into single word tokens (e.g., +"%Y" → +%Y as one token)
• Array indices test: Rewrote to avoid spec format newline ambiguity

Test plan

• Removed 5 skip markers (neg_array_indices_empty, neg_brace_no_expand_space, echo_escape_r, date_combined_format, sort_merge)
• cargo test --all-features passes (69 pass, 49 ignored)
• cargo test --test spec_tests passes (all 13 test groups including bash_comparison_tests)
• cargo clippy --all-targets --all-features -- -D warnings clean
• cargo fmt --check clean
• Updated specs/009-implementation-status.md and specs/005-builtins.md