Skip the renamed-params lookup in declared when nothing is renamed

[ericproulx]

Summary

DeclaredParamsHandler#build_memo_key looks every declared param up in the route's renamed params, keyed by the param's whole path. That key is built fresh each time (two Arrays and a String per declared param) on every declared call. Only an API using as: has anything in that table; everywhere else it is empty and every lookup misses. The lookup is now skipped when the table is empty.

Benchmarks

declared on five params (one nested Hash), isolated:

call	master	branch
declared(params, include_missing: false)	4.81 µs	4.29 µs (−11%)
declared(params)	6.16 µs	5.59 µs (−9%)

End to end, a JSON POST returning declared(params, include_missing: false) measured +1.4% (median of 7 interleaved subprocess rounds, Ruby 4.0.6, no JIT), which is inside that harness's noise floor: master against itself read −1.2% the same day. The saving is about 100 ns per declared param, so it is a small, fixed win that scales with how many params an endpoint declares rather than a headline number.

Test plan

• Full RSpec suite passes locally.
• RuboCop clean.
• Mutation-checked through existing specs: inverting the guard fails 14 as: specs.
• CI green.

🤖 Generated with Claude Code

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Danger Report

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[ericproulx]

Closing in favour of #2936, which combines #2922–#2934 into one PR, re-benchmarked as a whole against master. The write-up here (behaviour matrix, mutation results) still describes this part of the change.