feat(insertion-normalization): pin volatile reminder blocks so mid-history rewrites stop busting the cache

Claude Code re-serializes <system-reminder> hook blocks inside
otherwise-stable user messages later in the session — moving one into
its own message or merging it into a neighbour — which edits history
mid-prefix and re-bills everything after the edit (reported
independently as anthropics/claude-code#76606; measured here as the
splice/insert-mid class, ~40 kB re-billed per unmitigated hit on a
real session).

The extension keeps a per-conversation canonical model of the message
history keyed by content identity (message-hash.mjs: content hash +
occurrence ordinal — position-independent, so repeated identical
reminders stay distinct). Incoming volatile blocks are pinned to their
first-seen serialization: when CC re-shapes an old reminder, the
forwarded bytes keep the canonical form and the prefix survives. A
history that stops matching the model (compaction, true rewrites)
resets honestly rather than forcing a stale canon — pins survive the
reset, order assumptions do not.

Gated off by default: CACHE_FIX_INSERTION_NORMALIZE=1 enables
normalization, CACHE_FIX_VOLATILE_PIN=1 the pinning. State persists
under the state dir and survives proxy restarts.

Measured on live traffic (513-request session, 2026-07-28): every
observed splice/insert-mid pair forwarded with 0 re-billed bytes; the
canonical-order invariant, cross-request stability and sequence gates
all report 0 violations over 2.5 GB of captures.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: Gunther-Schulz

proxy/extensions.json

@@ -15,6 +15,7 @@
   "workflow-agent-id-synthesis": { "enabled": true, "order": 365 },
   "image-retry-circuit-breaker": { "enabled": true, "order": 370 },
   "read-dedupe": { "enabled": true, "order": 380 },
+  "insertion-normalization": { "enabled": true, "order": 395 },
   "cache-control-normalize": { "enabled": true, "order": 400 },
   "messages-cache-breakpoint": { "enabled": true, "order": 410 },
   "ttl-management": { "enabled": true, "order": 500 },