Read instance variables directly instead of evaluating Ruby source - #95

Summary

Read listed instance variables through the core getter, using PP.mcall, instead of compiling Ruby source with instance_eval. Objects overriding instance_eval currently print the override's result instead of their stored variable values.

Reproduction and verification

An Object with @x=123 and an instance_eval override returning :wrong currently renders @x=:wrong. Also checked overridden instance_variable_get, mixed symbol/string names, missing/nil/false variables and Unicode names across 500 objects. Local five-sample benchmark, 200 renders of 30 variables: about 16–21ms before, 6.4ms after; Ruby allocations roughly unchanged. No application-throughput claim.

• Isolated branch: 3,004 focused checks pass under rbenv Ruby 4.0.6.
• Existing upstream RBENV_VERSION=4.0.6 rbenv exec rake test: 36 tests / 56 assertions, zero failures before and after.
• 2,000 ordinary single-/multi-line outputs remain byte-identical.
• Source-only change; no test files added or modified under the contributor's task constraint. Focused scripts are outside the repository. Other Ruby engines/versions and native Windows were not executed.

Breaking-change notes

Listed names continue to mean instance-variable names. Core storage reads now ignore overrides of instance_eval/instance_variable_get; custom presentation belongs in pretty_print. Normal output is unchanged.