docs(phase-4): record linkedin-studio's two named requirements before the freeze

They named both on a live channel today, having withdrawn the half of their
original question that turned out not to bind them. Recorded here rather than in
a reply so the scope freeze has to answer them explicitly.

`write_index` ownership was genuinely ambiguous in this plan — they read
deliverable 4's "write" and asked which line owns it. It is deliverable 3, and
the plan now says so.

Their vocabulary requirement is already the Python half's shape:
`TypePolicy(allowed=None)` is the default and rewrites nothing, so there is no
DEFAULT_TYPE to snap to. The reference checker's collapse is a known deviation to
resolve with the convention owner under step 1, not a behaviour to lift.

docs/plan/phase-4-node-half.md

@@ -46,7 +46,40 @@ code.
      parity.
 3. **linkedin-studio stays plugin-local.** Its `ingest/published/`
    provenance-record grammar has a different lifecycle by design and is NOT
-   normalized into this library. Explicit non-goal, agreed up front.
+   normalized into this library. Explicit non-goal, agreed up front, and
+   confirmed by them on 2026-07-25 as their own fixed decision rather than
+   something conceded to us.
+
+   Two requirements they named on 2026-07-25, before the scope freeze, which
+   it must therefore answer explicitly rather than pass over. Both are about
+   not breaking what they have, not about building toward them:
+
+   - **`write_index` must have a do-nothing mode.** They run a fourth index
+     mode the spec does not describe: create-if-absent, no managed lines,
+     the whole file hand-editable. Their scaffold never opens an existing
+     index, so it cannot normalize one. The requirement is that this
+     library's writer match that — against an existing index carrying no
+     managed lines, write nothing, and do not normalize the frame "while it
+     is open anyway". This is the writer-side sibling of the Phase 3
+     decision that nothing enumerates a directory unless the profile says
+     the index is derived. `write_index` is deliverable 3, not deliverable
+     4's `write`; that was ambiguous in this plan and is now stated.
+   - **An absent vocabulary means "do not snap", never "snap to default".**
+     The reference checker collapses any non-matching type to a
+     `DEFAULT_TYPE`, which would erase their Profile / Operations /
+     PendingDiff distinction with zero byte change. Vocabulary is
+     per-bundle, never spec-global. This is already the Python half's shape
+     — `TypePolicy(allowed=None)` is the default and rewrites nothing — so
+     the Node half inherits a design rather than adopting a constraint, and
+     the collapse is a known deviation to resolve with the convention owner
+     under step 1 rather than lift.
+
+   They have offered a minimal tolerance fixture pair (a `brain/` hub whose
+   `index.md` carries `okf_version` as markdown text, and an
+   `ingest/published/` record: YAML-free five-line header terminated by
+   `\n---\n`, no trailing newline). Take the bytes rather than the
+   description — it is held as a tolerance case, never as a format this
+   library emits.
 4. **Persist gate = guard-as-contract.** There is no Node guard today. The Node
    half documents and enforces the guard's *contract* at its persist seam
    (fail-closed hook point, no content-safety logic of its own) so a future Node