feat(relations): Relation.sample(seed=) with validation sampling

[discreteds]

Summary

• Add a validated Relation.sample(n=, fraction=, seed=) contract and InvalidSampleArgumentsError.
• Thread seed through the public protocol, Mountainash backend protocol, and Polars/Narwhals/Ibis implementations.
• Clamp oversize n; support Narwhals lazy sampling through collect/sample/re-lazy; preserve Ibis Bernoulli semantics with strict xfails for ibis-polars and ibis-sqlite seed limitations.
• Keep Validator._slice on the execution backend, pin implicit seeds, record the effective fallback seed, and record a closed-by-default fallback when a non-empty input samples to zero rows.
• Compile Frictionless pattern checks through regex_contains, fixing false passes on Narwhals and ibis-duckdb.
• Retire the resolved NW-REL-03 divergence fact and generated census/docs entries.

Verification

• Targeted gate: 74 passed, 6 xfailed for the post-review validator/sample/capability checks.
• Pre-review targeted gate: 441 passed, 4 skipped, 14 xfailed.
• Full suite: 23051 passed, 81 skipped, 1 unrelated strict XPASS, 3469 xfailed.
• The unrelated XPASS is IB-DT-13 on ibis-sqlite in test_show_recent_errors; it is reproducible and filed as mountainash-central backlog item 105, not changed by this PR.
• Ruff clean on changed files.

Spec: mountainash-central 04.planning/mountainash/superpowers/specs/2026-08-17-syntheticdata-validation-roundtrip-design.md.
Part B is independent of this PR and of item 8j PR #297.