Replace bindings using a single name lookup

[OskarEichler]

Summary

Find the matching index directly, avoiding a second equality-based scan. Custom binding equality can currently make that second scan replace a different earlier binding.

Reproduction

require 'representable/json/hash'
type=Struct.new(:name) { def ==(_); true; end }
a=type.new('a'); old=type.new('b'); replacement=type.new('b')
map=Representable::Binding::Map.new([a,old]); map << replacement
p [map.first.equal?(a), map.last.equal?(replacement)]
# [false,false] before; [true,true] after

Verification

• Existing master suite on this isolated branch: 545 tests / 701 assertions, zero failures/errors, three existing skips on Ruby 3.2.11. Ruby 4.0.6 retains the same eight legacy inspection-format failures, zero errors and three skips. Existing tests/expectations are unchanged.
• 300 focused checks on Ruby 3.2.11/4.0.6 for this change. XML first-render uses a separate fresh-process check.
• Combined release-based verification with other focused fixes and attributed Use Setter in OverwriteOnNil if a :setter is defined #177/Hash value of null breaks parsing #264/Update MultiJSON usage to address deprecations #273 source backports: 544 tests / 700 assertions pass on Ruby 3.2.11, plus 4,410 focused checks and 36 boundary checks. Package retains 104 paths and unchanged compared metadata. All 36 runtime files compile on both Rubies; no newly introduced default Lint offenses.

Isolated YJIT measurement: three alternating process pairs, each with five batches of 1,000 last-entry replacements in a 1,000-binding map. Median time 0.063231→0.036169s, zero allocations in both measured loops. No production throughput claim.

Breaking changes / limitations

Replacement remains defined by name and preserves position. Append/replacement return values remain unchanged; misleading custom equality no longer redirects replacement.

No new/modified checked-in tests. Ruby 2.4–3.1, JRuby, Windows and Linux were not executed locally. Ruby >=2.4 and runtime dependency ranges remain unchanged. External test tooling supplies a legacy MiniTest alias and ruby-prof 1.7.x; no assertions were changed. No production verification is claimed.