Preserve variable flow through declare blocks

[drewmt]

Fixes phpstan/phpstan#15394.

Variables used inside a declare block are currently reported as unread. The inverse is also missed: assignments inside the block disappear from unused-variable analysis, including a block overwriting a value assigned outside it.

DeclareHandler already processes the nested statements, but drops their VariableFlow when returning the result. Forward that flow in both the PHP handler and its Turbo counterpart, leaving statement-only declarations unchanged. The separate native version bump follows the existing coupling requirement.

The regression covers reads inside a block, writes read afterwards, nested and alternative-syntax blocks, real unused writes, overwritten outer values, and statement-only declarations. It fails before the fix with three false positives and both expected real diagnostics missing.

Validation:

• All 34 UnusedVariableRuleTest cases pass.
• Full configured PHP 8.4 suites with and without activated Turbo: 22,452 tests, 96,880 assertions, 106 existing skips each; no failures. Full PHP 8.3 suite: 22,305 tests, 96,631 assertions, 204 existing skips; no failures.
• Strict native build, differential smoke tests, signature parity (16,390 members), and identical PHP/native traces on the declare fixtures pass.
• Full self-analysis in both modes, scoped coding standards, syntax and whitespace checks pass.
• The method/declaration checker reports the same generated-header self mismatch on the clean base and this branch; complete checker output is byte-identical. Those unrelated headers were not regenerated. Local clang-tidy 21 is unavailable; cross-platform and integration CI remain to be checked upstream.

[drewmt]

Rebased onto current 2.3.x and refreshed the Turbo pin; the 34 rule tests pass in both modes, along with the native build, smoke tests and signature parity. All twelve downstream failure diagnostic sets on the previous head also occur in the upstream pin-only PR #6696. New-head CI is running.

[drewmt]

Refreshed the rebase after the new Turbo pin conflict, preserving the original fix and upstream commits. Full suites pass in both modes (22,457 tests each), with the rebuilt native version, smoke and signature parity verified. New-head CI is running.

[drewmt]

Rebased onto 61f6fe5fb to resolve the native version-pin conflict, keeping the implementation and regression files unchanged. Fresh PHP 8.4 and activated Turbo suites both pass (22,475 tests, 96,906 assertions and 106 existing skips each), along with the native build, parity, self-analysis and coding-standard checks.