shorthand: rebuild the importance-stripped declaration

[samoht]

Stacked on #297 (chain: #295 → #297 → #298 → #299). Review the last two commits; the rest is the base.

without_importance built Declaration { r with important = false }, copying the cached hash field that was computed over (property, value, important) — so the copy carried a fingerprint of the importance it had just cleared. same_value compares with polymorphic =, which walks that field, so legacy_vendor_fallback's not (same_value ...) guard read two equal values as different:

a{display:-webkit-box;display:-webkit-box!important}
  before: a{display:-webkit-box;display:-webkit-box!important}
  after:  a{display:-webkit-box!important}

The equal-importance pair collapsed correctly all along; only the pair differing in importance leaked through.

Impact is output size — the wrong answer keeps a redundant declaration, and no mis-hashed declaration escapes the module — but a record update on a type whose smart constructor is meant to be the sole path is a landmine. Both arms now rebuild through Declaration.v and Declaration.theme_guarded, which recompute the hash; that is the shape Declaration.important already used two lines away.