feat!: lazy configuration for order-independent API

[lilith]

Summary

• All setter methods now buffer to a PendingConfig applied at start_compress() time in the correct order
• Fixes configuration ordering bugs where set_scan_optimization_mode() and set_fastest_defaults() silently reset other settings via internal jpeg_set_defaults() calls
• No compilation breakage: all existing methods preserved, deprecated methods emit warnings but still function

New APIs

• mutate_components_last() - modify components via a callback that runs after all other config
• mutate_cinfo_last() - access raw cinfo via a callback that runs after all other config
• set_force_8bit_quantization() - control 8-bit DQT clamping (applies to both quality and custom qtables)
• write_scanlines_strided() - write scanlines with custom stride for padded pixel data
• Validation in start_compress() returns io::Error instead of panicking for zero/oversized dimensions and invalid sampling factors

Breaking changes

• components() and cinfo() now take &mut self (were &self) — they apply pending config before returning so callers always see current state
• components_mut() → deprecated, use mutate_components_last()
• cinfo_mut() → deprecated, use mutate_cinfo_last()

Both deprecated methods still work and emit deprecation warnings.

The Bug This Fixes

// BROKEN in 0.10.x: smoothing gets reset to 0 comp.set_smoothing_factor(50); comp.set_scan_optimization_mode(ScanMode::Auto); // In 0.11.0, both orderings produce identical results

set_scan_optimization_mode(Auto) internally calls jpeg_simple_progression() which calls jpeg_set_defaults(), resetting quality, smoothing, subsampling, and more. Same for set_fastest_defaults().

Test plan

• 71 tests pass (31 unit + 40 integration)
• Combinatorial ordering tests verify all permutations produce identical output
• Edge case tests verify validation errors for invalid dimensions/sampling
• Deprecated API tests verify backward compatibility
• cargo clippy --all-targets --all-features -- -D warnings clean