refactor(builder): flashblock candidate build

[julio4]

Extract normal/continuous inner build in ONE primitive with:

• CandidateKind::{Empty, PoolBacked} (should candidate execute transaction for pool or not)
• InterruptPolicy::{JobOnly, JobAndFlashblock} (should candidate be cancelled on job or on flashblock/execution signals)
• shared prepare_candidate_budget helper

This kills duplication found in #534/#535/#546

[Copilot]

Pull request overview

Refactors flashblock candidate building by extracting shared logic into a single “candidate build” primitive, reducing duplicated code between the naive (non-continuous) and continuous builders while keeping their differing cancellation semantics and timing/metrics needs.

Changes:

• Introduces builder::candidate module with prepare_candidate_budget + build_candidate, including structured outcomes and timing breakdowns.
• Updates naive flashblock building to use the candidate primitive and records pool-fetch/execution timings via CandidateTimings.
• Updates continuous candidate loop to use the same primitive for both empty and pool-backed candidates, simplifying the loop and standardizing outcome handling.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
crates/op-rbuilder/src/builder/payload.rs	Switches naive flashblock build path to prepare_candidate_budget/build_candidate and wires timing-derived metrics recording.
crates/op-rbuilder/src/builder/mod.rs	Adds the new candidate module.
crates/op-rbuilder/src/builder/continuous/candidate_loop.rs	Replaces inlined candidate build logic with calls to the shared candidate primitive and adapts best-candidate handling.
crates/op-rbuilder/src/builder/candidate.rs	Adds the shared candidate-building implementation, outcome types, timing capture, interrupt policy handling, and unit tests.

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