fix: execution from non-zero instret

[jonathanpwang]

InterpretedInstance::execute_from_state for pure execution was using num_insns instead of instret_end in the ExecutionCtx constructor. This was not caught because we currently only ever execute from instret=0.

[Copilot]

Pull Request Overview

This PR fixes a bug in the execution context initialization where num_insns was incorrectly used instead of instret_end when executing from a non-zero instruction counter state.

• Properly calculates instret_end by adding num_insns to the current instret value
• Adds overflow protection with a new InstretOverflow error variant
• Moves instret retrieval to occur before execution context creation

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
crates/vm/src/arch/interpreter.rs	Fixes execution context initialization to use calculated instret_end instead of raw num_insns
crates/vm/src/arch/execution.rs	Adds new InstretOverflow error variant for handling instruction counter overflow

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[github-actions]

group	app.proof_time_ms	app.cycles	app.cells_used	leaf.proof_time_ms	leaf.cycles	leaf.cells_used
verify_fibair	(+4 [+1.8%]) 231	322,610	2,058,654	-	-	-
fibonacci	1,024	1,500,209	2,100,402	-	-	-
regex	(-24 [-1.0%]) 2,359	4,137,372	17,759,394	-	-	-
ecrecover	(+8 [+1.2%]) 702	122,859	2,262,718	-	-	-
pairing	(+8 [+0.6%]) 1,450	1,745,742	25,468,210	-	-	-

Commit: 2e0c069

Benchmark Workflow

[shuklaayush]

lgtm. good catch