[6.x] Fix blueprint fields being rebuilt for every entry

[daun]

TL;DR

• Problem: Loading many entries rebuilds the blueprint's fields for each, including references and imports
• Change: setParent() keeps the cached fields when the blueprint stays the same
• Result: 200 entries augment in 27ms instead of 525ms
• Fixes Warming stache (and everything else) taking a very long time since v5.38.0 #11189

Cause

Collection::entryBlueprint() calls setParent($entry) on one shared blueprint for every entry. For dated collections, Entry::hasTime() triggers this for each entry. resetFieldsCache() keeps the fields when the new parent has the same blueprint (#9585). But since #11116, setParent() calls resetBlueprintCache() first, which clears lastBlueprintHandle, so that check never matches.

In short, #11116 switched off the cache that #9585 added for exactly this case. This change keeps the fix from #11116.

Fix

• setParent() only calls resetFieldsCache()
• When the cache is kept, the new parent gets copies of the fields with their parent set to it. Copying instead of changing the cached fields means fields handed out earlier keep pointing at their own entry.
• Content changes (setContents(), ensureField(), …) still clear the cache.

Benchmarks

Stache

Same blueprint and 200 entries on the Stache. 5 runs. The fields are rebuilt when each entry is augmented.

	Before	This PR
Query only	7 ms	7 ms
Query + toAugmentedArray()	525 ms	27 ms

Eloquent

200 entries, dated collection, blueprint with 19 fieldset references and 2 imports. 5 runs.

	Before	This PR
Default entry class	547 ms	69 ms
Custom entry_class	768 ms	363 ms

Tests

Test	Before	After
Switching parents with the same blueprint doesn't rebuild fields	❌	✅
Fields handed out earlier keep their parent	✅	✅
Hydrating several dated entries resolves fields once	❌	✅

The test from #10061 still passes, as do tests/Fields, tests/Data/Entries and tests/Revisions. #9585, #11116 and #13430 added no tests.