chore: remove DenseRecordArena::current_size

[jonathanpwang]

The function is confusing and easy to misuse because it includes the initial offset bytes which are only for alignment correct purposes.

[github-actions]

group	app.proof_time_ms	app.cycles	app.cells_used	leaf.proof_time_ms	leaf.cycles	leaf.cells_used
verify_fibair	(+13 [+1.2%]) 1,120	322,658	17,671,904	-	-	-
fibonacci	(-11 [-0.5%]) 2,116	3,000,420	51,503,521	-	-	-
regex	(-123 [-1.7%]) 6,933	8,217,194	163,669,874	-	-	-
ecrecover	(-23 [-2.0%]) 1,111	274,568	8,596,011	-	-	-
pairing	3,640	3,725,930	93,105,105	-	-	-

Commit: ea6f170

Benchmark Workflow

[codspeed-hq]

CodSpeed WallTime Performance Report

Merging #1913 will not alter performance

_{Comparing chore/delete-cur-size (ea6f170) with feat/new-execution (2f03e2b)}

⚠️ 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

✅ 24 untouched benchmarks