fix: defer heavy module loading for faster startup (#272)

[esmuellert]

Summary

Reduces codediff.nvim's startup impact from ~15ms to ~0.6ms (96% reduction) by deferring heavy module loading until the user first invokes :CodeDiff.

Closes #272

Problem

plugin/codediff.lua eagerly loaded 42 Lua modules + a C library via FFI at startup, even when the user never used any diff functionality. This consumed ~15ms on cold start and ~8ms on warm start.

Before (startup profiling)

       startup: 48.9

codediff.lua            19.42   39.75 ███████████▊
codediff.ui             18.75   38.38 ███████████▌
codediff.ui.view        18.38   37.62 ███████████▎
codediff.ui.explorer     2.12    4.33 █▎
codediff.ui.auto_refresh 1.75    3.57 █
codediff.core.diff       1.09    2.24 ▌

After

       startup: 7.3

codediff.lua             0.58    7.92 ██▎
codediff.ui.highlights   0.31    4.32 █▎
codediff.config          0.18    2.47 ▌
codediff.core.virtual_file 0.09  1.20 ▎

Changes

• plugin/codediff.lua: Replace 4 eager top-level require() calls with 2 lightweight ones (codediff.ui.highlights + codediff.core.virtual_file). Wrap the :CodeDiff command handler to lazy-load codediff.commands on first invocation.
• plugin/vscode-diff.lua: Lazy-wrap the :VscodeDiff command handler.
• lua/codediff/ui/init.lua: Remove eager lifecycle.setup() call (was registering global autocmds at startup).
• lua/codediff/ui/view/init.lua: Add once-guard to call lifecycle.setup() on first view.create(), so cleanup autocmds are only registered when actually needed.

What loads at startup (~0.6ms)

• codediff.ui.highlights — highlight group definitions
• codediff.core.virtual_file — codediff:// URL scheme registration
• Command registration (negligible)

What's deferred to first :CodeDiff invocation

• C diff library (FFI + ffi.load())
• Explorer, History, Conflict UI modules
• Lifecycle autocmds (WinClosed/TabClosed/BufEnter)
• Git operations module
• View keymaps, navigation, layout

Testing

• ✅ All plenary tests pass (37 files, 0 failures)
• ✅ E2E validation: :CodeDiff file, :VscodeDiff, lifecycle cleanup, multi-tab all work
• ✅ 5 profiling runs confirm consistent ~0.6ms startup