sdk(P1.6): client-side head tracking — your SDK is a fork detector

Completes the verification chain (P1.2 -> P1.1 -> P1.3 -> P1.6). The client
remembers the last accepted (seq_no, event_hash) per stream and checks every
new receipt links forward; if the server rewinds a sequence number or presents
a divergent lineage, log_event / log_events raise AttestoForkDetected (Go:
*ForkDetectedError) and the stored head is NOT advanced. The customer's own
machine becomes the fork detector — no trust in any Attesto-side check.

- Python: HeadStore protocol + FileHeadStore (~/.attesto/heads.json, atomic,
  0600, default) + MemoryHeadStore; wired into sync and async v2 clients;
  head_store=None disables.
- TypeScript: HeadStore + MemoryHeadStore (default, edge-safe); Node-only
  FileHeadStore kept in a separate module (@attesto/sdk/heads-file) so the core
  bundle imports no node:fs; headStore: null disables.
- Go: HeadStore interface + MemoryHeadStore (default) + NewFileHeadStore;
  WithHeadStore option; WithHeadStore(nil) disables.

Same forward/rewind/divergence/gap semantics across all three (unit-tested:
in-order advance, forged-rewind fork, divergent-next fork, forward-gap accept,
file-store restart persistence). Existing v2 client tests pin head_store=None
(they replay overlapping seq). READMEs gain a "Your SDK is a witness" section.

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

README.md

@@ -112,6 +112,23 @@ comp := attesto.VerifyCompleteness(events, 5, 8)                     // no event
 numbers must be gap-free and each event's `prev_event_hash` must chain to the
 previous event's `event_hash`.
 
+## Your SDK is a witness
+
+The client remembers the last accepted `(seqNo, eventHash)` per stream and checks
+every new receipt links forward. If the server ever rewinds a sequence number or
+presents a divergent lineage, `LogEvent` / `LogEvents` return a
+`*ForkDetectedError` and the stored head is not advanced. The default store is
+in-memory; use a file store for fork detection across process invocations, or
+disable it.
+
+```go
+// Persist across CLI invocations (atomic, 0600 at ~/.attesto/heads.json):
+client, _ := attesto.NewClient(apiKey, attesto.WithHeadStore(attesto.NewFileHeadStore("")))
+
+// Disable fork detection:
+client, _ = attesto.NewClient(apiKey, attesto.WithHeadStore(nil))
+```
+
 ## Operator and Admin Endpoints
 
 System-key clients are created with `attesto.NewClient`. Tenant/operator