test: port comprehensive test cases from just-bash

[chaliy]

Summary

Significantly expand test coverage inspired by Vercel's just-bash project:

• AWK: 89 tests (was 19) - operators, loops, functions, printf formats
• SED: 65 tests (was 17) - addresses, ranges, regex, hold space
• JQ: 92 tests (was 21) - filters, functions, operators, string ops
• GREP: 55 tests (was 15) - flags, context, regex patterns

Also expand bash builtin tests:

• echo: 26 tests (escape sequences, flags, quoting)
• sort/uniq: 31 tests (numeric, reverse, field sorting)
• cut/tr: 35 tests (character/field modes, classes)
• wc: 22 tests (all counting modes)
• date: 31 tests (format specifiers)

Test Coverage Summary

Before	After
~281 total tests	510+ total tests
72 active in CI	301 active in CI
62 passing	168 passing
10 skipped	133 skipped

New tests marked with ### skip: document unimplemented features and serve as a roadmap for future work.

Test plan

• cargo test --features network passes
• cargo test --test spec_tests passes (all 4 categories: AWK, SED, JQ, GREP)
• cargo fmt --check passes
• cargo clippy passes
• Updated KNOWN_LIMITATIONS.md with new test counts

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