Dispatcher: cover table opcodes and :numeric_for (B5b-v2)

[davydog187]

Tracked by plan: .agents/plans/B5b-v2-dispatcher-tables.md

The plan file is the source of truth for goal, scope, success criteria, and implementation notes. Comments here are for discussion; the plan is for execution.

Why now

PR #237 (B5a-v2) shipped the dispatcher foundation: ~1.17x median speedup on fib(25). Profiling the full benchmark suite shows that every workload other than fib runs ~0% on the dispatcher today, because the bytecode encoder rejects the whole prototype on the first uncovered opcode and the table-shaped benchmarks all start with :new_table / :numeric_for / :set_table.

Coverage additions in this PR:

• :numeric_for (the loop primitive every benchmark relies on)
• :new_table, :get_table, :set_table, :set_field
• :set_list (basic non-multi-return form)
• :length (#t) — cheap, prevents single-opcode fallback

After this PR all four table_ops benchmarks compile end-to-end, plus the orchestrators in closures and string_ops.

When this work is picked up

1. The branch named in the plan's frontmatter is created.
2. A PR is opened with Closes #<this-issue> in the body.
3. Merge closes this issue.

For strategic context, see ROADMAP.md.