Optimize persistence I/O

[TypeThe0ry]

Summary

• track dirty revisions independently for use times, favourites, random placeholders, and custom placeholders
• write only changed SQL sections during automatic saves
• persist each cache through one pooled connection and one transaction, with rollback on any section failure
• write YAML through a same-directory temporary file and atomically replace the target when supported

Why

This is a focused follow-up to #1. The first round removes unconditional cache saves and smooths dispatch; this round reduces the remaining database connection/statement work and prevents a failed YAML overwrite from leaving a partially written data file.

Expected impact

• fewer SQL connection borrows and statements per autosave
• unchanged SQL sections no longer get rewritten
• related SQL sections commit as one unit
• interrupted YAML writes leave the previous data file intact

Validation

• git diff --check
• verified all runtime cache mutations map to a dirty section
• verified old global save-version call sites were removed
• GitHub Actions build and artifact upload passed: https://github.com/EllanServer/UltimateShop-AutoBuild/actions/runs/30683992631
• local compilation intentionally not run

Stack

This PR is stacked on #1 and intentionally targets codex/optimize-auto-save so its diff contains only the second optimization round.

[chatgpt-codex-connector]

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