Index parser lookups and cache aggregate diagnostics

[erseco]

Summary

• build immutable indexes for pages, blocks and iDevices after parsing
• make getPageById(), getBlockById() and getIdeviceById() constant-time lookups
• cache aggregate block/iDevice lists, page trees, orphan assets, broken references, internal links and package manifests
• cache asset-reference resolution against archive entries
• avoid duplicate scans of normalized standard-JSON iDevice state
• include normalized iDevice state when finding broken asset references
• record per-project timing, total elapsed time, peak memory and the five slowest upstream fixtures
• keep performance measurements informational rather than imposing brittle CI thresholds

Stack

This PR is stacked on #22 and targets feature/stream-inputs.

Tests

Adds repeated lookup/cache behavior checks and verifies that normalized standard-JSON state does not double-count asset occurrences.

CI is green on PHP 8.0–8.5, coverage, Composer validation, PHPCS and PHPStan.