feat(lang): more string and list built-ins (14 new operations)

[escapeboy]

Summary

• Adds 7 string built-ins: __builtin_string_contains, __builtin_string_starts_with, __builtin_string_ends_with, __builtin_string_to_upper, __builtin_string_to_lower, __builtin_string_trim, __builtin_string_join
• Adds 7 list built-ins: __builtin_list_len, __builtin_list_is_empty, __builtin_list_head, __builtin_list_tail, __builtin_list_append, __builtin_list_concat, __builtin_list_reverse
• Follows the __builtin_* pattern from feat(lang): string built-ins (int_to_string, float_to_string, string_len, string_chars) #59: opcodes in boruna-bytecode, VM dispatch in boruna-vm, typeck registration and codegen dispatch in boruna-compiler
• Opcodes assigned byte tags 0x8C–0x99 (continuing from 0x8B = StringChars in feat(lang): string built-ins (int_to_string, float_to_string, string_len, string_chars) #59)
• 21 new VM tests, all passing; clippy clean

Test plan

• cargo test -p boruna-compiler — 63 tests pass
• cargo test -p boruna-vm — 167 tests pass (includes 21 new built-in tests)
• cargo test -p boruna-tooling — 6 tests pass
• cargo clippy --workspace -- -D warnings — clean

🤖 Generated with Claude Code

[github-actions]

Bench compare

Threshold for regression: ≥ 10% slower mean.

Benchmark	Mean change	99% CI
compile_crud_admin_template	-2.06%	[-2.26%, -1.78%]
compile_medium_program	-3.57%	[-3.72%, -3.39%]
compile_small_program	-9.55%	[-9.76%, -9.37%]
evidence_build_5_steps	+3.13%	[+2.03%, +4.21%]
evidence_build_empty	+3.18%	[+2.28%, +4.07%]
evidence_verify_10_steps	+0.18%	[-0.67%, +1.12%]
evidence_verify_5_steps	-0.66%	[-9.94%, +5.69%]
vm_call_dispatch_loop/iters=1000	-2.11%	[-2.36%, -1.87%]
vm_call_dispatch_loop/iters=10000	-2.22%	[-2.29%, -2.16%]
vm_pure_loop/iters=1000	-4.28%	[-4.56%, -4.04%]
vm_pure_loop/iters=10000	-3.26%	[-3.43%, -3.07%]
vm_pure_loop/iters=100000	-3.52%	[-3.65%, -3.41%]
vm_record_loop/iters=1000	-2.05%	[-2.20%, -1.87%]
vm_record_loop/iters=10000	-2.71%	[-2.83%, -2.61%]

✅ No benchmark regressed past threshold.