[joe-sharp]

This resolves #1069. It builds on #1159 by @HeyNonster and the Zendesk Ruby infrastructure team. Their commit is cherry-picked as-is, with authorship and co-authors kept, and a follow-up commit addresses the review question there about what happens when _ already exists.

Changes on top of #1159

Don't clobber the debuggee's _. eval_binding is frame.binding.dup, and a duplicated binding shares the frame's locals. So local_variable_set(:_, ...) overwrites a _ the program already has, and the program keeps that value after continue:

def m; _ = 1; b = binding.dup; b.local_variable_set(:_, 99); p _; end; m
# => 99

Setting a name that doesn't exist yet only creates it on the dup, so that case is safe. Now _ is injected only when the original frame.binding doesn't define it (checking eval_binding would see the debugger's own _ after the first eval). When the program has its own _, it wins. This is the same idea as _return/_raised being renamed to %return/%raised when the program uses those names.

Only user evaluations update _. Previously every frame_eval updated the last value. That includes display expressions (which run at every stop), info ivars <expr>, ls <expr>, and DAP/CDP evaluations. Now only the p, pp and call eval paths update it. Plain REPL input goes through pp.

Tests

• test_display_does_not_update_underscore_local
• test_underscore_defined_by_program_is_not_clobbered: checks both what the debugger shows and what the program prints after continue.

rake test_console (310 tests) and rake test_protocol (64 tests) pass locally on Ruby 4.0.9.

IRB console mode

With irb_console enabled, a program-defined _ is still reset to nil. That reset comes from IRB, not from this change: IRB::WorkSpace#initialize runs @binding.local_variable_set(:_, nil), and IRB's debugger integration creates a new WorkSpace on the frame's binding at every prompt (Irb#debug_readline), which overwrites the existing variable.

That's fixed separately in ruby/irb#1266. With that branch on the load path, a debugger test for IRB console mode (a program-defined _ keeps its value across prompts and after continue) passes, along with the full test_console (311 tests) and test_protocol (64 tests) suites. That test isn't included here yet because it fails against released IRB. I can add it once ruby/irb#1266 is merged and released, or now if you'd rather it land alongside a version requirement.

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