v1.6.0

Adds two complementary observability surfaces to etag.Transport and
removes the single-purpose WithDriftDetected callback (BREAKING). Drift
transitions are now delivered via the unified WithEventCallback hook
as KindDriftDetected / KindDriftRecovered events; the full
DriftEvent payload remains available on evt.DriftEvent.

Added

• etag.Stats gains four per-Outcome counters: TotalHits,
  TotalMisses, TotalStores, TotalBypasses. All atomic, monotonic
  over the Transport's lifetime, read via Stats() without taking the
  driftMu mutex (the existing brief lock for {Degraded, DegradedAt}
  consistency is unchanged). Lets consumers compute hit-rate /
  store-rate / bypass-rate by polling Stats() at any cadence, without
  bumping the slog handler to LevelDebug to scrape etag_event records
  on the hot path. TotalBypasses aggregates all four uncached
  pass-through paths (bypass_oversize, bypass_noncacheable, both
  no_etag_header sites). Rare error and invalidation outcomes
  (get_error, store_error, remove_error, invalidated_gone) remain
  observable via slog at INFO/WARN level.
• etag.WithEventCallback(cb func(ctx context.Context, evt etag.Event))
  for per-call attribution. The callback fires on every cache decision,
  validation outcome, store/invalidation, and drift transition with the
  request URL, normalised path template, and Kind-specific fields.
• etag.Event struct and etag.Kind type with 14 constants matching
  the existing slog kind attribute values. Drift kinds drop the
  etag_ prefix that the slog kind attributes carry, for naming
  consistency with the bare etag_event kinds.

Removed (BREAKING)

• etag.WithDriftDetected(cb func(DriftEvent)) Option. Use
  etag.WithEventCallback(cb) and filter on
  evt.Kind == etag.KindDriftDetected || evt.Kind == etag.KindDriftRecovered.
  The full DriftEvent payload remains on evt.DriftEvent. See
  MIGRATION.md "v1.6: per-call event attribution" for a copy-paste swap.
• Slog event drift_callback_panic. The previous version used a
  recover() guard around the drift callback; v1.6 does not catch
  panics from WithEventCallback, so the corresponding slog event no
  longer fires. Panics propagate up through RoundTrip.

Documentation

• MIGRATION.md gains a "v1.6: per-call event attribution" recipe
  showing WithEventCallback ctx propagation, repo extraction from
  Event.URL, and the per-page callback fan-out under the pages
  package.
• README.md line 503 now references Stats's per-Outcome counters
  and the WithEventCallback hook.
• PROPOSALS.md updates the observability-roadmap row to mark
  per-call event callbacks as Implemented.
• Godoc on the Transport type (etag/transport.go:90-91) and on
  the Stats struct (etag/drift.go:55-56) updated to reference
  the new surfaces.
• Slog event allowlist comment at etag/transport.go:3-18 updated:
  drift_callback_panic removed (the recover guard is gone).
• etag/drift.go:1-9 package prologue updated: drift transitions are
  observable via WithEventCallback.