Tuning shop: perf optimizations for result fetching

[jeremy]

A series of profiling-guided optimizations to reduce object allocation and speed up hot paths in result processing, inspired by Trilogy's approach:

Allocation reductions:

• Skip field name allocation in as: :array mode (names aren't used)
• Eliminate unnecessary Hash allocation when no query options passed
• Lazy-load uncommonly accessed server flags
• Cache query options to avoid per-row lookups

Fast-path optimizations:

• Integer parsing fast path (Trilogy-style)
• cast: false fast path
• Encoding cache to avoid repeated rb_enc_find_index lookups
• force_encoding: "utf-8" option to skip charset→encoding handling when db and app encodings match (53% faster on string-heavy queries)

Partial casting:

• New approach: eagerly cast cheap types (integers, floats) while deferring expensive ones (datetime, decimal) for lazy access—best of both worlds

Benchmarks: 17-53% faster depending on query shape and options.

[jeremy]

This is an omnibus branch will all perf tunings. I'll split these out into distinct PRs for individual consideration.

[sodabrew]

Looking good!!