[DanielDarrenJones]

Fieldtype::config($key) is called constantly while augmenting and rendering, and every call rebuilt the whole config as collections: each config field's default value, merged with the field's own config, even when asking for a single key.

This changes it so that:

• Single-key reads return the field's own value, or that config field's default, directly.
• Each default is worked out once per config fields collection (the one Blink already keeps per fieldtype for the request), and only when it's first needed, so defaults for keys the field sets itself are still never computed.
• Computed (computed:) defaults are still resolved on every read.
• Reading the whole config gives the same array, in the same order, as before.

HasSelectOptions::getLabel() also finds the option with a loop rather than a filtered collection.

Benchmark

A generated site on 6.40.0 (entries, structured collections, navs of three sizes), six warm scenarios: three sets of page renders and three nav renders. Each request runs in its own PHP process, as PHP-FPM serves it (boot not timed, opcache on), on an M1 Pro with PHP 8.5. The rendered HTML is byte-identical to stock for two different generated sites.

This change on its own is 11% faster (geometric mean over the six scenarios).

Tests

Two new tests in FieldtypeTest:

• A config field's default is computed once across reads, and again after Blink is flushed. This fails without the change.
• Computed defaults are resolved on every read.

The existing it_only_computes_default_values_for_config_fields_missing_from_the_raw_config still passes. The full suite passes.