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.

src/server/compaction.ts

@@ -34,6 +34,7 @@ import {
 	buildCompactionPrompt,
 	compactionCutIndex,
 	COMPACTION_RESERVE_TOKENS,
+	estimateContextTokens,
 	estimateTokens,
 	newCompactionMarker,
 	planTreeToText,
@@ -142,13 +143,19 @@ export function effectiveThresholdTokens(settingTokens: number | undefined, cont
 
 // ─── the estimate over the ACTIVE projection ────────────────────────────────
 
-/** The projected context estimate: what the next LLM call would carry —
- *  toLlmMessages output (cut honored) + the system prompt. The record
- *  path carries no leading system message (settleWith filters it), so
- *  the chrome rides as the additive term instead: prompt text here,
- *  tool declarations absorbed by the 16k reserve (the declarations are
- *  a few k and stable per session). chars/4. */
+/** The projected context estimate: what the next LLM call would carry.
+ *  THE TRIGGER LAW (pi's, verified 2026-10-05): anchor on the last
+ *  assistant message's provider-reported usage — it already counted
+ *  the system prompt, tool declarations, and the provider's own
+ *  tokenizer — and chars/4-estimate ONLY messages appended after it.
+ *  estimateContextTokens refuses anchors that predate a compaction
+ *  marker (that usage described context the next request will NOT
+ *  carry), so the fallback path runs the plain heuristic over the
+ *  CUT-HONORED projection (never the dropped span) plus the additive
+ *  prompt term the anchor would have covered. */
 export function projectedContextTokens(messages: readonly LhMessage[], systemPromptTokens: number): number {
+	const est = estimateContextTokens(messages);
+	if (est.lastUsageIndex !== null) return est.tokens;
 	return estimateTokens(toLlmMessages(messages as readonly AgentMessage[]) as unknown as readonly LhMessage[]) + systemPromptTokens;
 }