compaction: the usage-anchored trigger + visible session state (round 15)

The relational-databases incident, diagnosed and fixed:

TRIGGER — pi's estimator law, adopted exactly: anchor on the last
assistant message's provider-reported usage (totalTokens || the field
sum) and chars/4-estimate ONLY what was appended after it. The anchor
scan skips aborted/zero-usage assistants and STOPS at the newest
compaction marker (pre-cut usage described context the next request
will not carry — pi's distrust rule). The old estimator
(JSON.stringify of every message /4) tripled the true context
(measured: 84,412 'tokens' estimated vs 38,799 provider-reported on
the incident record) — scaffolding, escaped LaTeX, thinking blocks
and app-side details all counted. estimateMessageTokens now counts
wire text only (text + thinking + toolCall name/args; content text
for user/toolResult/custom). The unanchored fallback runs the plain
heuristic over the CUT-HONORED projection plus the additive prompt
term; the anchored path adds nothing (usage already billed prompt
and tools).

OBSERVABILITY — compaction is a visible session-wide state (owner
spec): the transcript-bottom placeholder register gains COMPACTING
CONTEXT… (BrailleSpinner + Shimmer, the THINKING placeholder's twin,
mutually exclusive by construction — the gate precedes beginTurn),
and the sidebar row shows 'Compacting context · <ago>' with the
spinner (sessionMeta outranks busy; compacting-changed joins
turn-started/turn-settled as refresh triggers). Liveness rides the
API like turn-busy: ChatListItem.compacting + compactionInFlight on
both GET /api/chats and GET /api/chats/[id] (initiallyCompacting
covers the navigate-back-mid-compaction mount — the event already
fired), exported through the server barrel.

Also: the relational-databases record rolled back to pre-compaction
(marker, post-marker turns, tree reset; backup in /tmp) for a clean
retest. The summarizer-usage 'gap' was a misread — steps[] was
populated all along.

app/api/chats/[id]/route.ts

@@ -1,4 +1,4 @@
-import { deleteChatSession, getChat, isTurnRunning, sessionCoverage, stores, type ChatDeleteMode } from "@/src/server";
+import { compactionInFlight, deleteChatSession, getChat, isTurnRunning, sessionCoverage, stores, type ChatDeleteMode } from "@/src/server";
 
 export async function GET(_req: Request, { params }: { params: Promise<{ id: string }> }) {
 	const { id } = await params;
@@ -18,6 +18,7 @@ export async function GET(_req: Request, { params }: { params: Promise<{ id: str
 		...chat,
 		messages: chat.messages,
 		turnInFlight: isTurnRunning(id),
+		compacting: compactionInFlight(id),
 		coverage: sessionCoverage(id),
 	});
 }

app/api/chats/route.ts

@@ -1,6 +1,6 @@
 import { chainsForSelection, formatChainLine } from "@/src/core";
 import { newGraphContextMessage, type GraphContextPick } from "@/src/core";
-import { createChat, isTurnRunning, listChats, readSettingsRecord, stores, titleFromPrompt } from "@/src/server";
+import { compactionInFlight, createChat, isTurnRunning, listChats, readSettingsRecord, stores, titleFromPrompt } from "@/src/server";
 import type { ChatListItem } from "@/src/core";
 
 /** The session picker's list: record-derived summaries (phase, pending
@@ -14,6 +14,7 @@ export async function GET() {
 	const chats: ChatListItem[] = listChats().map((c) => ({
 		...c,
 		busy: isTurnRunning(c.id),
+		compacting: compactionInFlight(c.id),
 	}));
 	return Response.json({ chats });
 }

app/chat/[id]/page.tsx

@@ -1,7 +1,7 @@
 import { notFound } from "next/navigation";
 import { Chat } from "@/components/chat";
 import { slotsConfigured } from "@/src/core";
-import { getChat, isTurnRunning, readSettingsRecord } from "@/src/server";
+import { compactionInFlight, getChat, isTurnRunning, readSettingsRecord } from "@/src/server";
 
 export const dynamic = "force-dynamic";
 
@@ -34,6 +34,7 @@ export default async function ChatPage({
 			// A teacher turn that outlived its client: the remounted view must
 			// attach (busy shimmer + poll), never resurrect the stale record.
 			initiallyInFlight={isTurnRunning(chat.id)}
+			initiallyCompacting={compactionInFlight(chat.id)}
 		/>
 	);
 }