Simplify continuation L2G trace construction

[MauroToscano]

This PR keeps only the prove_epoch L2G staging cleanup.

Changes

• Build the epoch-local L2G trace directly from the cross-epoch boundary.
• Remove the temporary traces.local_to_global assignment and immediate mem::replace with an empty trace.
• Leave CLI epoch-size policy unchanged; no --epoch-size-log2 upper bound is introduced here.

Why

The L2G table is appended to the epoch proof explicitly, not via VmAirs::air_trace_pairs. Building it directly avoids constructing and swapping an unused temporary trace without changing the committed table.

Testing

• make compile-programs-asm
• cargo test -p lambda-vm-prover continuation::tests -- --nocapture
• make lint