fix: correct the conditions in requires.rs and return granular error

[snawaz]

As discussed in #94 (comment), this PR fixes the conditions in requires.rs (which was copied verbatim from loaders.rs — fixed that as well).

Summary by CodeRabbit

• Bug Fixes
  • Standardized errors when attempting to write to read-only accounts: now returns “Immutable” instead of a generic invalid data error, providing clearer feedback.
  • Consistent behavior across account loading and PDA-related operations.
• Refactor
  • Simplified writable checks for improved clarity, also correcting the behavior for valid writable accounts.

[snawaz]

• refactor: do not require system_program in instruction-accounts and add docs #102
• cleanup: remove older impl: delegate, commit_*, finalize, undelegate #103
• fix: correct the conditions in requires.rs and return granular error #101 👈 (View in Graphite)
• main

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[GabrielePicco]

LGTM!