refactor: reduce page executor complexity

[douglasmonsky]

CK-QG1A exact-main reapplication only.

Source authority:

• Exact base: 62d6cc1
• Authority packet: docs/roadmap/tasks/ck-qg1a-correct-page-executor-complexity.md
• Supersession authority: docs/decisions/evidence/ckqg1a0/page-executor-source-supersession-authority.json
• Permitted successor source digest: 9e80c8677dd4ceadc4fbd66681aedef78528b1ad4f50edc7a04f4b1c7ac12f31

Scope:

• One file only: src/codex_usage_tracker/agent_kernel/query/page_executor.py
• Extracts request identity and SHA-256 validation into a private helper, preserving validation order, exception types, messages, and all subsequent request/cursor/order/query-only behavior.
• No baseline, threshold, exemption, shared authority, query admission, EvidenceService, publication, or PR CK-QG1: enforce agent-kernel maintainability ratchet #392 changes.

Evidence:

• Frozen baseline digest: c490d954a5e9d09c61f884d51e3b9d3196af5615887f409c36f8469d1b2b6cf9
• Xenon findings resolved: PageExecutionRequest D/23 to C/17; PageExecutionRequest.post_init D/22 to C/16; helper B/7; no new or worsened finding.
• Focused page-executor, evidence, and service tests: 17 passed.
• Exact frozen-baseline C/B/B check: passed.
• just v: passed, 1368 tests and all checks.
• just vc: passed, including package build and distribution safety.
• GitNexus impact/detect_changes: low risk, one changed file, zero affected processes.
• One independent read-only reviewer: no material findings, safe to commit.

This PR is the single CK-QG1A correction required before any downstream task resumes.