Add configurable Grafast cache limits

[Zetazzz]

Summary

Implements planning item F06 on main, where the reusable performance harness is already available.

• adds explicit per-schema limits for Grafast query, operation, and operation-plan caches
• accepts limits from pgpm.json/runtime options or three environment variables
• validates every merged configuration source as a safe integer of at least 2
• installs one schema plugin only when at least one limit is configured
• preserves Grafast upstream defaults and current behavior when omitted
• keeps this PR independent of F03 preset-composition work and all scoped-introspection/retirement changes

Configuration

{
  "graphile": {
    "grafastCache": {
      "queryCacheMaxLength": 512,
      "operationsCacheMaxLength": 256,
      "operationOperationPlansCacheMaxLength": 32
    }
  }
}

Equivalent environment variables:

• GRAPHILE_QUERY_CACHE_MAX_LENGTH
• GRAPHILE_OPERATIONS_CACHE_MAX_LENGTH
• GRAPHILE_OPERATION_PLANS_CACHE_MAX_LENGTH

Validation

Rebased onto main@7d28f718c049. Verified final head 67f47e488de11fe6a21d2c4327b2ceb3ef780983:

• Full CI: all 17 jobs passed
• Examples Integration: passed
• Frozen-lockfile installation and git diff --check passed.

Local verification: 5 affected or restored packages built successfully; cache configuration and plugin tests passed 2 suites / 25 tests, including eviction behavior.

Refs constructive-io/constructive-planning#1693.