Replace tomasvotruba/unused-public with shipmonk/dead-code-detector

[TomasVotruba]

Swaps the dev dependency tomasvotruba/unused-public for shipmonk/dead-code-detector (^1.4), which detects unused methods, properties and constants.

Changes:

• composer.json: drop tomasvotruba/unused-public, add shipmonk/dead-code-detector (auto-registered via extension-installer).
• phpstan.neon: remove the unused_public block; set shipmonkDeadCode.cache.offloadCollectorData: false so cold-cache parallel runs do not crash with a missing DCD cache file.
• phpstan-baseline-dead-code.neon: baseline for the 11 initial findings, left for a follow-up cleanup. These include known false positives - the dynamically dispatched BetterStandardPrinter::p* printer methods and contract interface methods.

CI (composer complete-check) passes locally: phpstan level 8 green on a cold cache, ECS clean.