feat: add --timing flag wtih per-phase timing (v0.2.1)

[masonlet]

Summary

Adds --timing flag as an independent diagnostic option

Changes

• DiagnosticFlags for future diagnostic flags
• crate::time! macro in src/utils/timing.rs for timing blocks of code
• timing: bool threaded through all timed functions
• mono_repo_mode and single_repo_mode both print [timing] Total
• timing persisted to ConfigEntry and resolved from config defaults
• Show timing? prompt added to interactive mode
• ResolvedDiagnosticFlags replaces raw DiagnosticFlags in ResolvedArgs

Timing coverage

• Prerequisites check
• Git clone/update
• Build system detection
• Wrap hoisting (Meson)
• Config file generation
• Build directory creation/clean
• CMake configure + build
• Meson setup + compile
• Total elapsed per mode