linux: fix readlinkat guest copy length handling

[jlinenkohl]

Summary

• Fix readlinkat guest-copy path to use bounded copy semantics and correct signed-length handling.

Why

• Prevents guest memory copy errors and incorrect return handling on readlinkat.

Validation

• cd tests && bash run_unit_tests.sh -R test_elf

Depends on

• none

Stack Context

• Base runtime fix for relocation-related sequence.
• Recommended before elf: apply R_X86_64_RELATIVE addend semantics #68 and elf: add .relr.dyn bootstrap relocation support #69; required for strict green path leading into tests: split elf coverage into no-reloc control and reloc gate #70.

Test Evidence

• Date: 2026-04-02
• Branch-level validation source: phase14_audit baseline sweep
• Full unit harness: cd tests && bash run_unit_tests.sh -> 8/8 passed
• Integration tinytest lane: (cd guest/tests && bash build.sh) && ./build/tinytest guest/tests/glibc_test -> passed

PR-Scoped Command

• cd tests && bash run_unit_tests.sh -R test_elf
• Expected standalone status: green (base runtime fix)

[fwsGonzo]

Thanks!