fix(async-context-compression): reverse-unfolding to prevent progress drift

- Reconstruct native tool-calling sequences using reverse-unfolding mechanism
- Strictly use atomic grouping for safe native tool output trimming
- Add comprehensive test coverage for unfolding logic and issue drafts
- READMEs and docs synced (v1.4.1)

.agent/learnings/async-context-compression-progress-mapping.md

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+# Async Context Compression Progress Mapping
+
+> Discovered: 2026-03-10
+
+## Context
+Applies to `plugins/filters/async-context-compression/async_context_compression.py` once the inlet has already replaced early history with a synthetic summary message.
+
+## Finding
+`compressed_message_count` cannot be recalculated from the visible message list length after compression. Once a summary marker is present, the visible list mixes:
+- preserved head messages that are still before the saved boundary
+- one synthetic summary message
+- tail messages that map to original history starting at the saved boundary
+
+## Solution / Pattern
+Store the original-history boundary on the injected summary message metadata, then recover future progress using:
+- `original_count = covered_until + len(messages_after_summary_marker)`
+- `target_progress = max(covered_until, original_count - keep_last)`
+
+When the summary-model window is too small, trim newest atomic groups from the summary input so the saved boundary still matches what the summary actually covers.
+
+## Gotchas
+- If you trim from the head of the summary input, the saved progress can overstate coverage and hide messages that were never summarized.
+- Status previews for the next context must convert the saved original-history boundary back into the current visible view before rebuilding head/summary/tail.
+- `inlet(body["messages"])` and `outlet(body["messages"])` can both represent the full conversation while using different serializations:
+	- inlet may receive expanded native tool-call chains (`assistant(tool_calls) -> tool -> assistant`)
+	- outlet may receive a compact top-level transcript where tool calls are folded into assistant `<details type="tool_calls">` blocks
+- These two views do not share a safe `compressed_message_count` coordinate system. If outlet is in the compact assistant/details view, do not persist summary progress derived from its top-level message count.