Implement Prometheus proposal 0052 (extended range selectors) for the
thanos promql-engine. The anchored modifier pins boundary values to real
samples, while the smoothed modifier interpolates boundary values,
eliminating extrapolation artifacts in rate/increase/delta calculations.

Closes thanos-io#630

Changes across the engine layers:

- engine: add EnableExtendedRangeSelectors option, set parser flag
- logicalplan: extend storage time ranges for anchored/smoothed selectors
- execution: enforce function whitelists (anchored: rate, increase, delta,
  resets, changes; smoothed: rate, increase, delta), extend hint ranges
- ringbuffer: add Anchored/Smoothed fields to FunctionArgs, implement
  extendedRangeRate with pickOrInterpolateLeft/Right boundary logic and
  counter-reset correction
- storage/prometheus: thread anchored/smoothed through matrixSelector,
  use ext-lookback buffers, extend maxt for smoothed post-range samples

Constraint: parser.EnableExtendedRangeSelectors is a global variable in
prometheus/parser — can only be set to true (additive) to avoid races
Rejected: Streaming ring buffers for anchored/smoothed | they lack
ext-lookback and boundary interpolation support
Confidence: high
Scope-risk: narrow
Not-tested: native histograms with anchored/smoothed (rejected upstream too)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Sylvain Rabot <sylvain@abstraction.fr>

When step intervals don't align with sample intervals, the sample
needed as the ext lookback boundary may be trapped in lastSample from
the previous step and not pushed back to the buffer (since its timestamp
is <= mint). For anchored/smoothed selectors, push lastSample back into
the buffer before Reset so the ext lookback retention logic can decide
whether to keep it. This is scoped to anchored/smoothed only to avoid
changing existing x-function behavior.

Also adds a fuzz test (FuzzAnchoredSmoothedModifiers) that generates
random data and step parameters, testing all anchored/smoothed function
combinations against the upstream Prometheus engine. The fuzzer found
this bug within seconds.

Signed-off-by: Sylvain Rabot <sylvain@abstraction.fr>
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

… fix anchored/smoothed gaps

- Replace "proposal 0052" references with actual GitHub link to
  prometheus/proposals/0052-extended-range-selectors-semantics.md
- Remove extended_vectors.test from acceptance test skip list and
  enable EnableExtendedRangeSelectors in TestPromqlAcceptance
- Fix error message format to match Prometheus ("can only be used
  with: X, Y - not with Z") and defer validation errors to eval time
  via deferredError operator for expect-fail-msg compatibility
- Implement smoothed instant vector interpolation (selectPointSmoothed)
  matching Prometheus smoothSeries behaviour
- Fix anchored changes/resets on sparse data by retaining an additional
  lookback sample in the ring buffer and adding pickFirstSampleIndex
  to start comparisons from the correct boundary sample

Constraint: error messages must match Prometheus exactly for expect-fail-msg tests
Constraint: smoothed instant vectors need forward-looking samples for interpolation
Rejected: extending mint in matrix selector for anchored | would over-widen sample window for all functions
Confidence: high
Scope-risk: moderate
Not-tested: smoothed instant vectors with native histograms (returns no data, not an error)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Sylvain Rabot <sylvain@abstraction.fr>

Signed-off-by: Sylvain Rabot <sylvain@abstraction.fr>