A small register virtual machine, in the spirit of SQLite's VDBE (vdbe.c). ExSQL.Executor walks the AST directly; for the common single-table scan/filter/project shape it instead compiles the statement to a flat opcode program (see ExSQL.Executor.compile_vdbe/3) and runs it here, avoiding the per-row environment construction and recursive expression descent of the tree walker.

A program is a tuple of opcodes addressed by program counter. State threads through a single tail-recursive loop:

• one cursor over the table's rows (Rewind/Next advance it),
• an integer-keyed register file the opcodes read and write,
• the emitted output rows plus LIMIT/OFFSET counters.

Opcodes (each a tuple tagged by its name):

• {:rewind, end_pc} — position the cursor on the first row, or jump to end_pc when the table is empty.
• {:next, top_pc} — advance the cursor and jump to top_pc, or fall through when exhausted.
• {:column, col_key, reg} — load the cursor row's column into reg.
• {:rowid, reg} — load the cursor row's rowid into reg.
• {:value, value, reg} — load a constant into reg.
• {:eval, fun, reg} — load fun.(row, rowid) into reg. The closure is built by the compiler over the tree walker's evaluator, so arbitrary projection/sort expressions reuse exact SQL semantics.
• {:cmp, op, reg_a, aff_a, reg_b, aff_b, collation, fail_pc} — apply comparison affinity then compare; jump to fail_pc unless the result is true (NULL and false both fail, as in a WHERE clause).
• {:filter, fun, fail_pc} — jump to fail_pc unless fun.(row, rowid) is true; the general WHERE form when the :cmp chain doesn't apply.
• {:result_row, proj_regs, key_regs} — emit a row from proj_regs. With no ORDER BY (key_regs == []) rows stream out honoring OFFSET/LIMIT; with ORDER BY every row is collected with its key_regs sort key, then sorted and clamped after the scan.
• {:halt} — stop.

The comparison opcodes delegate to ExSQL.Value, the same module the tree walker uses, so affinity, collation, and NULL semantics match exactly.