fix validate_share panic after on_set_new_prev_hash in custom-work mode

[rx18-eng]

Body

Closes #2155.

• new JobStore::deactivate_job method that moves active_job into past_jobs
• ExtendedChannel::on_set_new_prev_hash false arm now calls deactivate_job() before mark_past_jobs_as_stale(), so the old active goes through past then stale and late shares for it return Stale instead of panicking
• StandardChannel::on_set_new_prev_hash reordered so job_id_to_target.clear() only runs after activate_future_job(...) returns true, so the error path can't corrupt state
• both expect("job target must exist") sites in validate_share (extended.rs:675, standard.rs:597) replaced with ok_or(ShareValidationError::InvalidJobId)?

Two new tests

• test_set_new_prev_hash_without_future_jobs_marks_active_as_stale in extended.rs reproduces the panic from the issue and asserts Err(Stale) after the fix
• test_set_new_prev_hash_without_future_jobs_preserves_state in standard.rs confirms state survives the error and the next share validates without panic

Both tests fail with the original panic on unpatched code; full crate suite passes (74/74) after the fix.

[rx18-eng]

@plebhash would appreciate a review from you !

[rx18-eng]

I fixed the failing semver check !