EngineConfig.instructions was Vec<PathBuf>, which forces embedders that
compute instructions at runtime to stage content to a disk file just to
satisfy the path API. That has two awkward side-effects:
1. The disk file looks like editable config but gets overwritten on
every launch, confusing for users browsing the install dir.
2. Multi-engine setups need per-engine paths to avoid `rehydrate`
re-reading the wrong session's content across concurrent engines.
Add an `InstructionSource` enum (`File(PathBuf)` / `Inline { name,
content }`) covering both shapes. `render_instructions_block` dispatches:
File sources read disk (original behavior preserved), Inline sources use
the content directly with `name` becoming the `<instructions source=...>`
attribute.
`From<PathBuf> for InstructionSource` is provided so the existing CLI /
runtime call sites upgrade with a single `.into_iter().map(Into::into).
collect()` chain — no behavior change for callers passing paths.
New test `render_instructions_block_handles_inline_source` covers Inline
empty / oversize-truncate / File+Inline mixed-ordering. The 5 existing
`render_instructions_block_*` tests are updated to use `.into()` on
`PathBuf` and continue to assert File-variant behavior.
hexin
3aea6d29a2