[tisonkun]

Why

KLL and REQ reported empty distributions as InvalidArgument, while T-Digest returned None but panicked on invalid query arguments. This change separates absent results from invalid requests, following the discussion in #266 and the implementation in #273.

Design

• Validate query arguments first. Invalid arguments return InvalidArgument even when the sketch is empty.
• Return a meaningful result when one exists; otherwise return None. Queries that also validate arguments return Result<Option<T>, Error>.

These principles are recorded briefly in CONTRIBUTING.md. Each method documents its own empty-result and error contract.

API changes

API	Return type
KLL / REQ rank	Option<f64>
KLL / REQ quantile, quantiles, cdf, pmf	Result<Option<_>, Error>
TDigest / TDigestMut rank, quantile, quantiles, cdf, pmf	Result<Option<_>, Error>

The changes include the queries exposed by KLL/REQ sorted views. Valid queries on empty data return None or Ok(None); callers of fallible queries can use ? before handling the optional result.

• Ranks outside [0, 1], NaN arguments, and invalid split-point ordering produce errors where applicable.
• On non-empty data, quantiles(&[]) returns Ok(Some(vec![])); cdf(&[]) and pmf(&[]) return Ok(Some(vec![1.0])). Empty data returns Ok(None).
• The audit of the other sketch families found meaningful zero, false, or empty results, or existing Option contracts such as extrema and intersection initialization. Those APIs retain their behavior.
• The changelog describes the REQ and T-Digest migrations. KLL is unreleased. Numeric algorithms and serialization formats are preserved.

Commits

1. 5bda675 fix(kll): distinguish empty queries from invalid arguments
2. fd871f9 fix(req): distinguish empty queries from invalid arguments
3. 6947ee0 fix(tdigest): distinguish empty queries from invalid arguments
4. d938a1b docs: keep query contracts on their methods
5. 4e0ecc6 docs: show imports in doctest examples

The first three commits each cover one sketch family. Documentation cleanup and visible doctest imports are separate commits. Hidden trailing Ok(...) expressions remain available for examples using ?.

Validation

• cargo x check
• cargo x test, including 144 doctests and cross-language serialization tests
• cargo x lint

Coverage includes empty, singleton, and compressed states; invalid arguments before emptiness; empty query slices; valid infinite T-Digest rank/split-point arguments; and existing endpoint and large-weight regressions.

Credit

Adapted from @Phoenix500526's work in #273. The three implementation commits retain:

Co-authored-by: Jiawei Zhao <Phoenix500526@163.com>