[FIX] Restore PHP 8.3 compatibility

[Dmi3yy]

Problem

eMCP currently declares PHP 8.4 as the minimum runtime, which blocks installation in PHP 8.3 environments even though the upstream MCP runtime already supports PHP 8.3.

Why

The current package baseline is stricter than both Evolution and the supported laravel/mcp ^0.5.x runtime window. That makes the package harder to adopt as a base Evolution extra without providing any clear code-level benefit.

What Changed

• lowered the root package requirement from php ^8.4 to php ^8.3
• pinned the supported upstream runtime to laravel/mcp ^0.5.9
• updated README and DOCS requirements in English and Ukrainian
• aligned PRD, SPEC, and upstream-smoke notes with the PHP 8.3 baseline and 0.5.9 minimum
• updated the existing CI workflow to run on PHP 8.3 instead of 8.4

Where

• composer.json
• README.md
• README.uk.md
• DOCS.md
• DOCS.uk.md
• PRD.md
• SPEC.md
• docs/spikes/upstream-smoke.md
• .github/workflows/ci.yml

Verification

• composer validate --strict
• php tests/Phase6/UpstreamAdapterSmokeTest.php
• php tests/Phase6/DocsConfigCommandsConsistencyTest.php
• php tests/Phase6/CiWorkflowCoverageTest.php
• sh scripts/lint-php.sh
• isolated Composer resolution with platform.php=8.3.0, which resolves successfully and selects laravel/mcp v0.5.9

Remaining Gaps

• I did not widen the supported upstream window to 0.6.x; this keeps the change minimal and within the current documented ^0.5.x policy
• no functional runtime behavior changed here; this PR only narrows the package baseline to the already-supported upstream/runtime floor

[middleDuckAi]

Superseded by #2. This earlier PR was opened under the wrong GitHub identity during setup correction; the valid MiddleDuck PR is now #2 from middleDuckAi:middleDuck/emcp-php83-compat-md.