refactor(diff): clarify path parameter names in show_diff (#55)

[AzarAI-TOP]

Closes #55.

What

Renames the path-like parameters threaded through lua/code-preview/diff.lua so their names match what they actually hold:

Old	New	Meaning
real_file_path	display_path	the winbar display string (often a cwd-relative path)
abs_file_path	file_path	the canonical identity — active_diffs key, passed to the changes registry / neo-tree reveal
original_path	original_source_path	temp file holding the before content
proposed_path	proposed_source_path	temp file holding the after content

So the signature becomes:

M.show_diff(original_source_path, proposed_source_path, display_path, file_path, action, backend)

Scope

Pure rename, no behaviour change. Touches the four functions that thread these args:
mark_change_and_reveal, build_inline_diff, show_inline_diff, and M.show_diff — plus two now-stale comments in tests/plugin/diff_lifecycle_spec.lua.

A couple of notes vs. the issue description, since the code has moved since it was filed:

• The show_diff call sites now live in lua/code-preview/pre_tool/init.lua and already pass display_path(...) / file_path positionally, so they need no change.
• bin/core-pre-tool.sh referenced in the issue no longer exists on main; there are no remaining references to the old names anywhere in the repo (grep clean).

Acceptance criteria

• All references to real_file_path and abs_file_path removed in favour of display_path and file_path
• All references to original_path / proposed_path (temp content files) renamed to *_source_path
• Existing tests pass with no behaviour change

Testing

./tests/run_lua.sh — all plugin specs pass: 9 files, 0 failures, 0 errors (incl. diff_lifecycle_spec.lua, 18/18). Neovim v0.12.