fix(precompile-storage): replace unimplemented! panics with structured errors in SetHandler transient methods

[eric-ships]

BOP-286

Motivation

SetHandler<T> implemented the Handler<T> transient methods (t_read, t_write, t_delete) with unimplemented!(), causing a process panic if any generic caller invoked them. The Handler<T> trait returns Result precisely so callers can handle unsupported operations without crashing.

The practical risk is a node crash rather than a clean revert: a future developer adding a Set field to a transient storage struct, or a generic caller iterating over handler methods, would cause an unrecoverable panic at runtime instead of receiving a structured error. Even though transient Set storage is unlikely by design, the fix is a 3-line change that permanently closes this crash vector. The panic vs. Err(Fatal) distinction matters for operational stability: Fatal surfaces to revm's frame handler, while a panic kills the process.

What changed

Replaced the three unimplemented!() stubs in SetHandler<T>'s Handler impl with Err(BasePrecompileError::Fatal("Set does not support transient storage".into())). Added a test asserting all three methods return Err.

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1

[linear]

BOP-286

[github-actions]

Review Summary

No issues found. This is a clean, well-motivated change.

The replacement of unimplemented!() with Err(BasePrecompileError::Fatal(...)) is the correct fix:

• Fatal is the appropriate error variant — it's documented as "Unrecoverable internal error" and is consistent with how Set::store() already handles its own unsupported operation (line 129).
• All other Handler implementations (Slot, VecHandler, ArrayHandler, BytesLikeHandler) provide real transient storage support by delegating to Slot's TransientOps. SetHandler is the only one that cannot support transient storage due to its dual-structure invariants (values Vec + positions Mapping), so explicitly returning an error is the right approach.
• The test covers all three methods and uses is_err() which is sufficient for this case.

[github-actions]

✅ base-std fork tests: all 602 passed

base/base is fully in sync with the base-std spec.