docs(changelog): Unreleased — cheap connect, Diagnostics tab, tool status

Written on dev as part of finishing the merge, per the convention adopted
2026-09-18: feature branches do not touch CHANGELOG.md, and the entry describes
what actually landed rather than what a branch intended.

First time through the new way rather than discovering the conflict afterward —
the merge was clean.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qcu9ByJfuKBQxmrWb8rSrN

CHANGELOG.md

@@ -4,6 +4,52 @@ All notable changes to seismo-relay are documented here.
 
 ---
 
+## Unreleased
+
+### Added
+
+- **Diagnostics tab in the SFM standalone webapp.**  Surfaces the device
+  endpoints that previously existed only as `curl`: `events/storage_range` and
+  `events/index` alongside `monitor/status`, then stop monitoring, disable ACH
+  (`rescue?erase=false`, so stored events survive), and erase.  The wedged-unit
+  ladder — slow drip and blind stop — sits under its own heading pointing at
+  `docs/runbooks/wedged_unit_recovery.md`, with the reminder that `slow_drip`'s
+  success signal is `bytes_received > 0` and not a clean duration.  Erase is
+  guarded by typing the unit's serial: auth answers *who*, not *did you mean
+  it*, and Swagger's try-it-out button on `/device/events/erase` is live on
+  `:8200/docs`.
+
+- **`docs/sfm_tool_status.md`** — an honest per-capability maturity assessment:
+  what is production-grade (the codec library, the data side), what is
+  emergency-grade (the device side), what is a research artifact, the
+  known-issues table, and the gap to a real tool.  Also records the **5A
+  page-boundary bug** as known: `parse_strt_end_offset()` discards the key's
+  page byte, so once a unit has recorded more than 64 KB since its last erase,
+  an event spanning the boundary reads an `end_offset` *behind* its own start —
+  the chunk loop fetches nothing and TERM packs a negative `offset_word`, which
+  500s.  Reproduced on BE12599.  Production is unaffected: it ingests complete
+  files via the watcher path and never runs this walk.
+
+### Changed
+
+- **Connecting to a unit no longer walks its event chain.**  `/device/events`
+  reads every event header over the cellular link; on a unit with a large or
+  wrapped chain that takes minutes or fails outright, and it fired
+  automatically on every connect.  Connect now uses only ~2 s probes —
+  `/device/info` (which already carried the compliance config the walk was
+  re-reading) plus `events/storage_range` — and the Device tab gains an Event
+  Chain card.  The walk moved behind a **Load events** button in the Events
+  toolbar.  Knowing whether a unit's ACH is on no longer requires reading every
+  event it has stored.
+
+### Migration
+
+**None.**  Frontend and documentation only — no codec, waveform-store or DB
+change, no schema change, and no `TOOL_VERSION` bump.  The webapp is served
+from the image, so the change appears after the next `sfm` rebuild.
+
+---
+
 ## v0.31.0 — 2026-09-18
 
 **Report parity, and a second way to rescue a runaway unit.**  Two threads.