Preserve translation text outside valid capture placeholders

[OskarEichler]

Summary

Only substitute canonical positive translation capture references, and preserve out-of-range references as literal text. Static and dynamic translation should not crash or silently remove strings such as URL-encoded %20 when they do not refer to a capture.

Reproduction

With the translator plugin enabled and an identity tr_fn:

p 100%20 complete
p 100%20 complete #{"yes"}
p %0 and #{"yes"}

Before: static mode crashes on %20; dynamic mode drops it when an interpolation exists; %0 incorrectly selects the last capture. After: %20 and %0 remain literal in both modes. Existing valid %1, %2, etc. capture references continue to work.

Verification

• Unmodified core/plugin suites: 415 runs / 650 assertions, zero failures/errors/skips on Ruby 3.2.11 and 4.0.6; existing Rails suite 15 runs / 36 assertions on each.
• 3,200 focused checks per Ruby across static/dynamic modes, zero/one capture, normal text, zero/leading-zero/out-of-range tokens.
• Syntax passes and no new comparative Lint offenses; no test files changed.

Compatibility / limitations

No dependency or API change. %0 and leading-zero references no longer alias real captures, and nonexistent references remain literal instead of disappearing or raising. Literal text that collides with an actual %1 reference remains ambiguous; this does not introduce a new escaping convention or change translation catalogs.