Avoid duplicate L2G trace work in continuations

[MauroToscano]

Summary

• Build continuation epoch traces once and reuse the touched-cell set recorded by that trace build.
• Stop generating a throwaway genesis-only L2G trace inside build_traces; the continuation driver now installs the real provenance-aware boundary trace.
• Keep epoch_touched_cells available for tests/helpers, but remove it from the hot continuation proving loop.

Rationale

The continuation prover was replaying each epoch's logs once to collect touched cells, then replaying them again to build traces. The trace build already has the final MemoryState, so it can expose the sparse touched-cell set directly.

build_traces also generated an L2G trace that production immediately overwrote with the cross-epoch provenance-aware boundary. This PR avoids that dead work and leaves L2G construction to the continuation driver.

Verification

• cargo fmt --all
• make compile-programs-asm CLANG=/opt/homebrew/opt/llvm/bin/clang
• cargo test -p lambda-vm-prover continuation -- --nocapture
• cargo test -p lambda-vm-prover local_to_global -- --nocapture
• make lint