[PHP 8.6] Test scaffolding and deprecation-clean audit

[lisachenko]

Refs #220 (part of epic #219).

Establishes the PHP 8.6 test scaffolding, audits the code base against the "Deprecations for PHP 8.6" RFC batch, and makes the experimental 8.6 CI row readable. No library behaviour changes — src/ is untouched.

1. Stub / guard scaffolding

• tests/Stub/FileWithClasses86.php — baseline 8.6 stub (interface, trait, abstract class, final class with a promoted property and a variadic union-typed method, backed enum, namespaced function and constant). It deliberately uses only syntax that is already valid and parseable, so per-feature 8.6 stubs (PFA, readonly defaults, #[\Override] on constants, …) can be added later as separate FileWith<Feature>86.php files by their own tickets.
• AbstractTestCase::getFilesToAnalyze() — yields the stub behind PHP_VERSION_ID >= 80600. The guard is load-bearing: the parity data providers include_once every stub they yield, so a *86.php stub must never reach an older runtime.
• tests/Php86ParsingTest.php — proves the wiring in both directions. The static assertions (file parsing, class/enum discovery, methods, parameters, types, constants, traits) run on every runtime, because the engine analyzes sources without loading them; the native-reflection parity test is guarded with PHP_VERSION_ID >= 80600, and the "reflect by name without loading" test is guarded the other way, since the stub is loaded by the parity providers on 8.6.
• CLAUDE.md — documents both conventions for future contributors.

On 8.6 the stub also flows through the whole existing parity matrix, which is where the extra ~1090 tests come from.

2. Deprecation audit

Checked src/ and tests/ against the accepted items of the Deprecations for PHP 8.6 RFC: is_double()/is_integer()/is_long()/doubleval(), return from finally, list(), let/is/readonly/_ as identifiers, namespace as a class constant, strcoll(), SORT_LOCALE_STRING, metaphone(), spl_classes(), spl_object_hash(), SplFileObject CSV methods, mysqli_stmt_init()/mysqli_get_charset(), case-insensitive define(), is_a()/is_subclass_of() with a string subject and allow_string = false, objects passed to array_walk()/array_walk_recursive()/mb_convert_variables()/http_build_query()/deflate_init()/inflate_init(), session_set_save_handler() without create_sid()/validateId(), ArrayIterator inheriting ArrayObject implementations, and the Reflection items (ReflectionProperty::setValue()/setRawValue() with wrong types, ReflectionMethod::invoke()/invokeArgs() with an object for a static method).

Result: no hits. Details worth recording:

• The three finally blocks in the code base (src/Resolver/TypeExpressionResolver.php:96, src/Resolver/NodeExpressionResolver.php:190, tests/ConstantExpressionClosuresTest.php:217) only perform cleanup and do not return.
• The single is_subclass_of() call (src/Traits/ReflectionClassLikeTrait.php:853) passes an object, not a string, so it is unaffected.
• ReflectionProperty::setValue() (src/ReflectionProperty.php:763) already normalises its arguments to null/object before delegating to the parent, so it cannot trigger the new wrong-type deprecation.
• All define() usages in the stubs are two-argument calls.

Verified empirically as well: the full suite on 8.6 with --fail-on-deprecation --fail-on-notice --fail-on-warning --display-deprecations reports nothing (phpunit.xml.dist already sets error_reporting=E_ALL).

3. CI

The experimental 8.6 job was verified against the last master run (run 32818096739): both 8.6 rows install via setup-php and really execute the suite on the beta (--ignore-platform-req=php+ correctly relaxes only the upper platform bound). Nothing was broken, so the changes are cosmetic but do address the "surface the result clearly" part of the ticket:

• The job name now reads PHPUnit tests (PHP 8.6, highest deps) [experimental] instead of PHPUnit tests (8.6, highest, ubuntu-latest, true).
• Because continue-on-error reports a failing experimental row as successful in the checks list, the outcome of the test step is now written to the run summary for experimental rows.
• The 8.6 rows keep continue-on-error: true until GA, as required.

Local results

Check	Result
vendor/bin/phpunit (PHP 8.5.9)	OK — 13749 tests, 15424 assertions, 134 skipped, 2 incomplete (baseline: 13743 / 133 / 2)
php8.6 vendor/bin/phpunit (PHP 8.6.0beta1)	OK — 14842 tests, 16608 assertions, 145 skipped, 3 incomplete; also green with --fail-on-deprecation --fail-on-notice --fail-on-warning
vendor/bin/phpstan analyse src --no-progress (level 10)	[OK] No errors

Finding for a follow-up (not fixed here — out of scope)

While drafting the stub, an unrelated pre-existing parity gap surfaced in ReflectionMethod::getPrototype() (src/ReflectionMethod.php:281). It searches the parent class first and does not resolve transitively, so for

interface I { public function describe(): string; }
abstract class A implements I { abstract public function describe(): string; }
final class C extends A { public function describe(): string { return ''; } }

it returns A while native reflection returns I. This is not 8.6-specific and would be a behaviour change in src/, so the stub was written to avoid the construct instead; it deserves its own ticket.

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