feat: Implement Phase 2 Feed Formats - ATOM, JSON Feed, RSS fix (Phases 2.0-2.3)

This commit implements the first three phases of v1.1.2 Phase 2 Feed Formats,
adding ATOM 1.0 and JSON Feed 1.1 support alongside the existing RSS feed.

CRITICAL BUG FIX:
- Fixed RSS streaming feed ordering (was showing oldest-first instead of newest-first)
- Streaming RSS removed incorrect reversed() call at line 198
- Feedgen RSS kept correct reversed() to compensate for library behavior

NEW FEATURES:
- ATOM 1.0 feed generation (RFC 4287 compliant)
  - Proper XML namespacing and RFC 3339 dates
  - Streaming and non-streaming methods
  - 11 comprehensive tests

- JSON Feed 1.1 generation (JSON Feed spec compliant)
  - RFC 3339 dates and UTF-8 JSON output
  - Custom _starpunk extension with permalink_path and word_count
  - 13 comprehensive tests

REFACTORING:
- Restructured feed code into starpunk/feeds/ module
  - feeds/rss.py - RSS 2.0 (moved from feed.py)
  - feeds/atom.py - ATOM 1.0 (new)
  - feeds/json_feed.py - JSON Feed 1.1 (new)
- Backward compatible feed.py shim for existing imports
- Business metrics integrated into all feed generators

TESTING:
- Created shared test helper tests/helpers/feed_ordering.py
- Helper validates newest-first ordering across all formats
- 48 total feed tests, all passing
  - RSS: 24 tests
  - ATOM: 11 tests
  - JSON Feed: 13 tests

FILES CHANGED:
- Modified: starpunk/feed.py (now compatibility shim)
- New: starpunk/feeds/ module with rss.py, atom.py, json_feed.py
- New: tests/helpers/feed_ordering.py (shared test helper)
- New: tests/test_feeds_atom.py, tests/test_feeds_json.py
- Modified: CHANGELOG.md (Phase 2 entries)
- New: docs/reports/2025-11-26-v1.1.2-phase2-feed-formats-partial.md

NEXT STEPS:
Phase 2.4 (Content Negotiation) pending - will add /feed endpoint with
Accept header negotiation and explicit format endpoints.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

tests/test_feed.py

@@ -23,6 +23,7 @@ from starpunk.feed import (
 )
 from starpunk.notes import create_note
 from starpunk.models import Note
+from tests.helpers.feed_ordering import assert_feed_newest_first
 
 
 @pytest.fixture
@@ -134,7 +135,7 @@ class TestGenerateFeed:
             assert len(items) == 3
 
     def test_generate_feed_newest_first(self, app):
-        """Test feed displays notes in newest-first order"""
+        """Test feed displays notes in newest-first order (regression test for v1.1.2)"""
         with app.app_context():
             # Create notes with distinct timestamps (oldest to newest in creation order)
             import time
@@ -161,6 +162,10 @@ class TestGenerateFeed:
                 notes=notes,
             )
 
+            # Use shared helper to verify ordering
+            assert_feed_newest_first(feed_xml, format_type='rss', expected_count=3)
+
+            # Also verify manually with XML parsing
             root = ET.fromstring(feed_xml)
             channel = root.find("channel")
             items = channel.findall("item")