docs(stream-install): mark one-pass provider complete

NEXT_STEPS item 2 still described the old per-file unrar provider shape even
though the current shared provider now performs one technical listing pass and
one sequential unrar payload pass per archive. Update the roadmap so the next
implementation slice starts at the remaining solid-archive policy work instead
of chasing an already-replaced extraction loop.

The item 3 wording now keeps the solid/non-solid archive fork explicit without
suggesting the current provider still needs to be swapped merely to avoid
per-file extraction.

Test Plan:
- git diff --check

Refs: NEXT_STEPS.md item 2

NEXT_STEPS.md

@@ -13,22 +13,25 @@ product-ready.
    and hands fetched file details to `StreamInstallGame` instead of the normal
    download command.
 
-2. **Replace per-file `unrar p` with a final archive provider**
+2. **Done — Replace per-file `unrar p` with a final archive provider**
 
-   The prototype provider is intentionally simple: `unrar lt`, then `unrar p`
-   per file. Good for non-solid archives, bad for solid archives. Final shape
-   should be a one-pass provider with real entry boundaries, likely via libunrar
-   or a purpose-built wrapper.
+   The shared external `unrar` stream provider now runs `unrar lt` once for the
+   archive metadata and one sequential `unrar p` pass per archive for payload
+   bytes. It frames directories, file starts, file chunks, and file ends from
+   the technical listing, so CLI and GUI callers use one purpose-built provider
+   instead of a per-file extraction loop.
 
 3. **Handle solid archives deliberately**
 
-   Add archive inspection that decides:
+   The provider exposes the RAR `solid` flag in `ArchiveBegin`; the remaining
+   work is to make that flag a tested policy decision instead of an incidental
+   stream attribute. Add archive inspection that decides:
 
-   - non-solid: per-file streaming is fine
-   - solid: one sequential archive pass only
+   - non-solid: current one-pass streaming is fine
+   - solid: prove and enforce one sequential archive pass only
 
-   This is the big architectural fork we discussed, and the prototype’s
-   provider is the thing to swap.
+   This is the big architectural fork we discussed: keep non-solid and solid
+   archive handling explicit in the provider contract and scenario coverage.
 
 4. **Decide the integrity model**