Reader: expand validation helper coverage for linker outputs

[katsyoshi]

Parent: #39

Remaining scope:

• Expand Reader-side validation for non-PLT relocation/tag cases where tests still rely on ad-hoc shell checks
• Add targeted failure messages for missing/invalid linked sections when optional relocation sections are present
• Migrate remaining linker tests to use Reader validation helper paths where practical

This tracks the unfinished part after PR #40.

[katsyoshi]

Completed by the Reader-side validation work in PR #40 and follow-up updates.\n\nImplemented scope includes:\n- validation of .rela.dyn range and DT_RELA/DT_RELASZ/DT_RELAENT consistency\n- validation of .rela.plt entry range/type (R_X86_64_JUMP_SLOT)\n- validation of DT_JMPREL/DT_PLTREL/DT_PLTRELSZ/DT_PLTGOT consistency against linked sections\n\nLinker tests now rely on Reader validation paths for these core checks.