feat(awk): implement phase 1 core interpreter

[matt-dz]

Summary

Implements the first practical awk slice on top of the rewritten harness, adding the registered awk builtin with core parsing and evaluation. The phase covers BEGIN/END, record and field reads, simple separators, regex/comparison patterns, arithmetic and string behavior, and the GNU awk oracle wiring needed to keep the implementation honest.

• Adds the builtins/awk implementation and registers the command.
• Supports core printing, fields, patterns, assignments, arithmetic, and string concatenation.
• Enables the first 107 rewritten AWK scenarios plus GNU awk oracle shell scenarios.
• Adds pinned GNU awk CI jobs and awk-focused fuzzing.

Validation

• make test_against_gawk
• make test_awk_rewritten
• tools/awk-harness/run.sh fuzz