CI: Report warnings scoped to files changed in the PR

## Problem

The current "Enforce warning reduction" CI step reports the total warning count across the entire project. When the project doesn't have zero warnings, it becomes painful for PR authors to determine **which warnings were actually introduced by their changes** versus which ones already existed in the codebase.

This makes the CI step feel bothersome rather than helpful, as developers have to manually cross-reference the full warning list against their changed files.

## Proposed Solution

Enhance the CI step to **report only the warnings that appear in files changed by the PR**. This would:

1. Parse the compiler warning output and extract file paths from each warning
2. Compare those file paths against the list of files changed in the PR (available via `git diff --name-only` against the base branch)
3. Report only the warnings that match changed files, making it immediately clear which warnings the PR author needs to address

### Expected Behavior

- The CI step should list warnings **scoped to the PR's changed files**, so authors can directly see what they need to fix
- Optionally, still show the total warning count delta (before vs after) as a summary, but the detailed list should be filtered to changed files only

### Additional Considerations

- This could be implemented as a PR comment (e.g., via a bot comment) that lists the relevant warnings inline
- Consider also highlighting **new** warnings (warnings that exist in the PR branch but not in the base branch for the same files) vs pre-existing ones

Requested by Nikita Khalov via Slack