refactor: align theta view and coupon naming

[tisonkun]

Summary

This PR applies the two approved naming changes from the follow-up audit:

• rename RawThetaSketchView::theta() to theta64()
• rename Coupon::from_hash() directly to from_value() (option 8B)

The cached seed_hash fields remain unchanged following the final decision on item 6. The raw-view visibility strategy (item 1) and changelog work (item 5) remain deferred.

Commit-by-commit rationale

1. 2aabb8a — Align the raw Theta threshold name. Renames RawThetaSketchView::theta() to theta64() and updates every implementation and set-operation call site. The suffix makes the raw 64-bit threshold representation explicit and avoids conflating it with higher-level theta terminology. This is a naming-only change; threshold values and algorithms are unchanged.
2. 5baab5e — Name the Coupon constructor after its input. Renames Coupon::from_hash(value) to Coupon::from_value(value) across the public API, documentation, examples, internal callers, and tests. The method hashes its generic input internally, so from_hash incorrectly implied that callers supplied an already-computed hash. Per option 8B, this is a direct rename without a deprecated compatibility alias.

Compatibility notes

Both commits rename public methods. The changelog entry is intentionally deferred until the release review requested in item 5.

Validation

• cargo x test
• cargo x lint
• git diff --check origin/main...HEAD
• rebased on the latest main (including refactor: align seed and internal naming #164)

[tisonkun]

Renames Coupon::from_hash(value) to Coupon::from_value(value). The method hashes its generic input internally, so from_hash incorrectly implied that callers supplied an already-computed hash.

cc @notfilippo is this rename reasonable?