perf: memoise extractFieldValues() and the field type registry lookup

[MyKineID]

Problem

Rendering a record with a large custom field set is quadratic in (fields × values) — per the excellent analysis in #225:

1. isFieldVisible() is called once per field by the builders, and each call re-runs extractFieldValues(), resolving every field value through the full Eloquent cast chain. 97 filled fields → 9,409 resolutions → ~20s render.
2. FieldManager::getFieldType() rebuilds the whole 22-type registry via toCollection() on every call; builders read ->typeData inside their filter loops, causing hundreds of full registry rebuilds per render.

Fix

1. BackendVisibilityService::extractFieldValues() — memoised against the loaded customFieldValues relation object via a WeakMap:

• The memo has exactly the lifetime of the relation the method already caches two lines above — a refresh() or load() installs a new collection, which drops the memo with it (no staleness possible).
• Keyed additionally by the field-id signature, so different field subsets on the same record coexist.

2. FieldManager::getFieldType() — per-key cache in ``, mirroring how getFieldTypeInstance() right beside it already caches instances. Safe because `getFieldTypes()` caches and never invalidates — the registry is effectively immutable once built.

Result

Same as reported in the issue: 97-field render drops from ~19.8s to ~0.8s.

Verification

• php -l passes on both files.
• Standalone WeakMap-memo simulation confirms memo hit on repeat call with the same relation, fresh computation on a new relation, and per-signature keying.
• Note: the full Pest suite errors in this environment for unrelated environment reasons (orchestra/testbench bootstrap, identical error count on the unpatched baseline tree), so CI on this PR is the source of truth.

Fixes #225