docs(changelog): reorganize Unreleased sections to match released-section style

Phase 2 changelog audit. Consolidate fragmented section headers (5x Added / 5x
Fixed / 4x Changed in main; duplicate Changed in tracker) into one of each,
introduce 2.6.0-style `####` functional subsections, and condense bullets from
PR-description density to 1-3 sentences matching prior releases. WI-IDs,
framework enumerations, and quantitative bakeoff-relevance claims preserved.
No content relocation (Phase 0) or completeness work (Phase 1) performed.

Signed-off-by: jgstern-agent <josh-agent@iterabloom.com>

Author: jgstern-agent

.ci/affected-tests.txt

@@ -1,8 +1,8 @@
 # Test selection manifest
-# Generated by smart-test at 2026-04-21T01:17:00-04:00
+# Generated by smart-test at 2026-04-21T03:11:01-04:00
 # Mode: targeted
 # Baseline: 7484e9d523ac1b77f43a4fe71a90c54c5aa6864d
-# Changed files: 7
+# Changed files: 8
 # Changed source files: 0
 0
 # Selected tests: 1