Add Reader-based validation for PLT relocations and dynamic tags

[katsyoshi]

Goal: reduce ad-hoc readelf checks by adding Reader-level validation helpers for PLT-related linkage data.

Scope:

• Validate rela.plt entries are structurally correct (symbol index, type, offset).
• Validate consistency of dynamic tags: DT_JMPREL, DT_PLTREL, DT_PLTRELSZ, DT_PLTGOT.
• Use helper(s) in linker tests (including plt-call test).

Acceptance criteria:

• Tests can assert PLT relocation/tag sanity without shelling out to readelf for core checks.
• Helper errors are specific enough to pinpoint broken fields.

[katsyoshi]

Tracking update:\n- #41 (completed, closed): Reader validation for .rela.plt + DT_PLT* consistency (implemented in PR #40)\n- #42 (open): remaining Reader validation helper expansion\n\nKeeping #39 open as the parent tracker.