Split reorderForCaching into a structured reorderAndSplitForCaching that
returns {before, after} at the semi-static→dynamic section boundary.
- prompt-ordering.js: export reorderAndSplitForCaching — returns null if no
dynamic sections, otherwise {before: static+semi-static, after: dynamic}
- auto.js: import and wire reorderAndSplitForCaching into deps
- phases-unit.js: use split function; pass promptParts to runUnit when split
succeeds; fall back to flat reorderForCaching when null
- run-unit.js: when promptParts is present, send a two-block content array
[{type:text, text:before, cache_control:{type:ephemeral}}, {type:text, text:after}]
so Anthropic-compatible providers cache the stable prefix
- openai-completions.ts: preserve cache_control on text parts in convertMessages;
skip maybeAddOpenRouterAnthropicCacheControl if any part already has cache_control
Tests: 5 new contract tests for reorderAndSplitForCaching; all 4502 unit tests pass.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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