Runtime configurable defaults

Prefer runtime-configurable values over hardcoded constants to allow users to customize behavior without recompilation. Use environment variables with os.getenv_opt() for runtime configuration instead of compile-time constants with $d(), and consider environment variables instead of complex CLI parsing.

Why this matters:

• Runtime configuration allows users to adjust behavior without recompiling
• Environment variables provide a standard, cross-platform way to customize settings
• Reduces the need for complex command-line argument parsing
• Makes tools more flexible for different use cases and environments

How to apply:

// Instead of hardcoded constants:
const indexexpr_cutoff = 10

// Use runtime-configurable defaults:
const indexexpr_cutoff = os.getenv_opt('VET_INDEXEXPR_CUTOFF') or { '10' }.int()

// For buffer sizes and similar values:
const buff_size = int($d('gg_text_buff_size', 2048))

// Instead of complex CLI parsing for tools:
// Use environment variables like VDIFF_TOOL, VTMP_DIR, etc.

This approach allows users to customize behavior with simple commands like VET_INDEXEXPR_CUTOFF=99 v vet . without requiring recompilation or complex flag combinations.