perf(vm): preallocate map literal HashMap with known pair count 🗺️

[timfennis]

Summary

• OpCode::MakeMap knows the exact pair count at execution time, so HashMap::with_capacity(pairs) avoids the regrow/rehash churn HashMap::new() would incur as keys get inserted.
• Adds benches/programs/map_literal_build.ndc so this path is actually exercised by cargo bench going forward — the existing map_ops bench only constructs %{}, which doesn't hit this code.

Measurement

Built release-with-debug binaries for master and this branch and compared with hyperfine --warmup 3 --runs 15 -N:

Benchmark	base	new	ratio
map_literal_build (new)	90.7 ms	63.2 ms	1.44× faster ± 0.09
map_ops (existing, %{} only)	25.3 ms	23.5 ms	within noise (1.07× ± 0.21)

The 32-entry-per-iteration literal in map_literal_build runs 50_000 times. Output (50000) matches on both binaries.

Test plan

• cargo test --release passes (unchanged result set)
• cargo fmt
• Output of new bench identical between base and new binaries
• hyperfine confirms speedup on populated map literals