chore: add 'allocated' method to DenseRecordArena

[arayikhalatyan]

Added a method called allocated to be used in axiom-gpu
Removed unnecessary trait requirements for metadata when implementing DenseRecordArena::alloc.

[codspeed-hq]

CodSpeed WallTime Performance Report

Merging #1785 will not alter performance

_{Comparing chore/densearena-allocated (9fd7f93) with feat/new-execution (29e11c3)}

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

Summary

✅ 20 untouched benchmarks

[github-actions]

group	app.proof_time_ms	app.cycles	app.cells_used	leaf.proof_time_ms	leaf.cycles	leaf.cells_used
verify_fibair	(-14 [-1.1%]) 1,262	322,699	17,340,154	-	-	-
fibonacci	(-12 [-0.5%]) 2,196	1,500,277	50,589,503	-	-	-
regex	(-8 [-0.1%]) 6,818	4,165,432	166,519,456	-	-	-
ecrecover	(-7 [-0.6%]) 1,114	137,260	8,151,353	-	-	-
pairing	3,729	1,862,964	97,508,739	-	-	-

Commit: 9fd7f93

Benchmark Workflow