Refactor: clean up path parameter naming in diff.show_diff

[Cannon07]

Motivation

lua/code-preview/diff.lua show_diff(original_path, proposed_path, real_file_path, abs_file_path, action) takes four path-like parameters whose names have drifted. Two are misleading:

• real_file_path is not the real path — it's the display string used in the winbar (often a cwd-relative path). The name implies it's the canonical identity.
• abs_file_path is the actual canonical identity (key in active_diffs, passed to the changes registry, used by neo-tree reveal).
• original_path / proposed_path are paths to temp files (/tmp/claude-diff-{original,proposed}-<pid>) holding the before/after content — not the real file.

A reader (or future-us during issue #47 phase 3) will misuse these. The friction shows up in diff.lua, bin/core-pre-tool.sh, and bin/apply-patch.lua, all of which thread these args through.

Proposed naming

The three concepts:

• Source paths — the two temp files: original_source_path, proposed_source_path.
• File path — the canonical absolute path of the real file (identity): file_path.
• Display path — what's shown in the winbar: display_path.

So: show_diff(original_source_path, proposed_source_path, display_path, file_path, action).

Scope

• Pure refactor. No behaviour change.
• Touches lua/code-preview/diff.lua (function signature + internal use), bin/core-pre-tool.sh and bin/apply-patch.lua (call sites, JSON arg keys).
• Tests under tests/plugin/diff_lifecycle_spec.lua may reference parameter names — update accordingly.

Out of scope

• Issue Migrate hook orchestration from bash to Lua #47 phase 3 / phase 4 — the bash→Lua port of core-pre-tool.sh. This rename should land before or after phase 3, not as part of it, so Migrate hook orchestration from bash to Lua #47's diff stays reviewable as a port (not a port + rename).

Acceptance

• 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 for temp content files renamed to _source_path.
• Existing tests pass with no behaviour change.