feat(awk): add Unicode \u escape sequences

[chaliy]

Summary

• Add \u escape sequence support in awk string literals (gawk 5.3+ feature)
• Supports 1-8 hex digits for BMP and supplementary plane characters (e.g., \u0041 → A, \u1F600 → 😀)
• Invalid code points or bare \u gracefully fall back to literal text
• Update builtins spec to document --csv/-k and \u escape support

Test plan

• 5 new unit tests for Unicode escapes (basic, multibyte, emoji, bare \u, mixed)
• 2 new spec tests for Unicode escapes
• All existing awk tests still pass
• Full test suite passes
• cargo clippy and cargo fmt clean

Closes #617