refactor(builtins): migrate sort, uniq, tr, tar, rg to ArgParser

[chaliy]

Summary

• Migrate 5 builtins (sort, uniq, tr, tar, rg) from manual while i < args.len() index loops to the shared ArgParser utility
• Add bool_flags() method to ArgParser for combined short boolean flags like -rnuf
• Net reduction: 196 insertions, 139 deletions (includes new tests)

What changed

ArgParser (arg_parser.rs): New bool_flags(allowed) method that consumes combined short boolean flags (e.g., -rnuf) when all chars are in the allowed set. 5 unit tests added.

Sort (sortuniq.rs): Replaced manual index loop with flag_value_opt for -t, -k, -o and bool_flags for -rnufscChMmz.

Uniq (sortuniq.rs): Replaced manual index loop with flag_value_opt for -f and bool_flags for -cdui.

Tr (cuttr.rs): Replaced manual for arg in ctx.args.iter() with bool_flags("dscC") + positional().

Tar (archive.rs): Replaced manual index loop with flag_value_opt for -f/-C, is_flag() + char iteration for combined groups like -czf.

Rg (rg.rs): Replaced manual nested char loops with flag_value for -m, explicit long flag matching, and char iteration for combined short flags.

Why

Closes #880 — manual arg parsing is repetitive, error-prone, and harder to read. ArgParser eliminates index arithmetic and bounds checking.

Test plan

• All 1934 existing unit tests pass (0 failures, 0 ignored)
• All integration tests pass (spec_cases, urandom_tests)
• 5 new bool_flags unit tests added (happy path, unknown char rejection, long flag, single dash, single char)
• Smoke tested: sort -n, tr a-z A-Z, cut -d, -f2, uniq -c
• cargo fmt --check clean
• cargo clippy --all-targets -- -D warnings clean