Require child success when matching all input

[OskarEichler]

Summary

The all combinator currently checks only how much input the child consumed. A failed child with zero length is therefore accepted when the input is empty. Also require a successful child result.

Reproduction

module SingleX
  include Raabro
  def root(i); str(:root, i, "x"); end
  def rewrite_root(t); t.string; end
end
SingleX.parse("")

Before: returns "". After: returns nil, as the grammar requires x.

Checked valid "x", input-offset rollback and an intentionally empty-string grammar, which still succeeds.

Verification

• Reviewed master baseline, each individual patch, and the combined installed-release branch: existing suite passes 94 tests / 169 assertions, zero failures.
• Ruby 4.0.6 via rbenv, Probatio 1.6.2. No new or modified test files.
• Focused temporary repros, Ruby syntax and gem packaging checks pass. Targeted Lint reports the same 13 pre-existing warnings on baseline and fixed source, with no new warnings.
• Consumer branch is based on installed 1.5.0 rather than importing unrelated master changes.

No production operations. Other Ruby/platform combinations were not run locally; no full supported-platform claim. Current open/closed issue/PR searches found no overlapping fix.

Breaking changes

Intentional correctness change: grammars that cannot match empty input no longer report success for it. Grammars that explicitly match empty input are unchanged. Error-diagnostic formatting is handled by a separate patch.