fix: keep extracted longhands from being reset by leftover shorthands

[dyk1454683243-sudo]

Problem

After restructure, a longhand that appears after a shorthand in the source can be pulled into a shared rule that sits before leftover shorthands. The later shorthand resets the longhand in the browser.

Minimal repro from #478:

.a {
    background: none;
    background-size: 1;
    b: c;
}
.b {
    background: some;
    background-size: 1;
    b: c;
}

Previously became:

.a,.b{background-size:1;b:c}.a{background:0 0}.b{background:some}

background then wipes background-size.

The same class of bug shows up with border / border-bottom (#417) and with all.

Root cause

8-restructRuleset extracts equal declarations into a new shared rule. For the usual merge-up path that rule is inserted before both leftovers. compareDeclarations only compares declaration identity — it does not know that background resets background-size.

Fix

• Add a shorthand → atomic longhand map (lib/restructure/propertyFamily.js), in the same spirit as the unfinished property-family branch.
• compareDeclarationsSafe refuses to extract a declaration when a leftover unique would sit on the other side of it and reset its atoms:
  • leftover shorthand after an extracted longhand (the A bug with restructuring and shorthands #478 case)
  • extracted shorthand after a leftover longhand (the Wrong restructuring of border rules #417 subset case)
• Unrelated equals still merge. Safe subset-before merges (background then background-size) are unchanged. Different !important flags make order irrelevant, so those longhands can still be extracted.
• all is treated as resetting every property except unicode-bidi and direction.

Tests

• fixtures/compress/restructure.merge/issue-478*.css — issue repro, shared unrelated props, subset after / before, !important, all
• fixtures/compress/restructure.merge/issue-417.css — border-bottom / border
• test/propertyFamily.js — overlap / reset helpers

npm test: 549 passing, 4 pending
npm run lint: clean

Fixes #478

Related: #196, #417

[dyk1454683243-sudo]

Withdrawing this PR while I clean up a high-volume open-PR backlog. Sorry for the noise — happy to come back later with a focused change if useful.