docs: REVERSE the streaming verdict — MessageDisplay hook makes it achievable

The previous commit on this branch concluded TUI streaming was impossible. That was
WRONG, and this corrects it before it could be merged.

The adversarial reviewer commissioned to refute the claim found, on a second pass while
verifying the fold-in, that its OWN first-pass hook enumeration had been truncated by a
400-char grep cap: it reported 21 hook events; the shipped 2.1.207 bundle has 30.
Event #30 is MessageDisplay.

Independently reproduced before acting on it (30 events confirmed via `strings` on the
binary; payload shape `hook_event_name:"MessageDisplay",turn_id,message_id,index,final,
delta`), then live-tested with a MessageDisplay command hook registered via --settings on
a PLAIN INTERACTIVE TUI spawn (no -p, no --bare), claude-sonnet-4-6, --effort low:

  banner: "Sonnet 4.6 with low effort · Claude Max"   ← subscription pool, verified

  7 fires, mid-turn, spread across generation:
    index=0 final=false  '## Mutex\n\n'
    index=1 final=false  'A **mutual exclusion lock** prevents concurrent access to a shar…'
    index=4 final=false  'let counter = 0;\n\nasync function increment() {\n  const release =…'
    index=6 final=true   '```'

  concat(deltas) === T (transcript-authoritative)  ->  TRUE  (579 == 579 bytes)
  T.startsWith(S) at EVERY step                    ->  TRUE  (prefix-stable)
  '## ' / '**' / '```javascript' present in deltas ->  raw markdown SOURCE, not rendered

This satisfies every invariant the previous version declared unobtainable: byte-faithful,
incremental, prefix-stable, no -p, subscription pool. Granularity is block-level (~5-7
chunks/answer), not token-level — which is all an SSE delta.content needs.

Backlog #2 REOPENS and should be built. Implementer caveat recorded: the hook's source
sets forceSyncExecution -> claude BLOCKS on it, so the hook must write and exit
immediately (FIFO/socket), never work inline. Only text blocks fire it (thinking excluded).
ALIGNMENT: consumes claude's OWN hook surface as emitted — forwarding, not inventing
(Class B / ADR 0007; no cli.js citation applies).

Everything still true is kept, and the dead ends are kept as dead ends (they document what
NOT to build): the pane is a rendered view whose source markers are irrecoverable
(capture-pane -e emits IDENTICAL SGR 1 for an H2 and a bold span — a provably non-unique
inverse); the transcript is event-granular; --debug-file carries timing but no payload;
--output-format stream-json requires -p (the metered pool). Also kept: the ~4s (n=1
same-turn) overhead correction, backlog #4's null result with its mechanistic single-user
reason, and the honest value framing — streaming moves the FIRST byte, not the last, so
the complete-answer consumer that motivated this work gains nothing from it.

The wrong conclusion and its refutation are both preserved in the doc. "We checked, it's
impossible" is the most expensive claim to get wrong: it closes a door nobody re-opens.

Docs-only. No code change, no version bump.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VqgWJcjxrjjL9L9SkpZyXR

docs/plans/2026-07-13-tui-latency/README.md

@@ -119,23 +119,28 @@ the effort level silently changes if the operator ever switches to env-token mod
 - **Risk**: none — banner confirms it stays on `Claude Max` (see `billing-banner.txt`).
 - ⚠️ Do **not** reach for `--bare` to shave boot: see above.
 
-### 2. Real streaming instead of blocking on turn-terminal — ~~the big one (~20 s)~~ → **DEAD**
+### 2. Real streaming instead of blocking on turn-terminal — **ACHIEVABLE → [`streaming-spike.md`](streaming-spike.md)**
 
-> **2026-07-13 update — the prereq spike below was run, and it kills this item.** The transcript
-> grows at *event* granularity (the whole answer lands in one line, ~0.3 s before terminal), and the
-> pane is a **rendered** view whose `capture-pane` text no longer contains the answer's source bytes
-> (`## `, `**`, code fences are gone) — so no pane-derived stream can be reconciled with the
-> authoritative text. A third source (`--debug-file`) does emit mid-turn stream events at
-> `CLAUDE_CODE_DEBUG_LOG_LEVEL=verbose` — but they carry **timing only, no text payload** (0
-> `text_delta` at any verbosity); its only byte-exact text is the end-of-turn `Stop` hook payload.
-> `--output-format stream-json`, the only interface that emits token deltas, requires `-p` — the
-> metered-billing path that TUI mode exists to avoid. **True token streaming is not achievable on the
-> TUI path**; OCP's TUI SSE is replay-only. Note also that streaming would never have shortened a
-> turn — only its first byte — so a consumer that needs the *complete* answer (the JSON-card case
-> that motivated this investigation) gains nothing from it.
+> **2026-07-13 update — the prereq spike was run. The answer is YES, but not from either source this
+> item guessed at.** (a) The transcript grows at *event* granularity (the whole answer lands in one
+> line, ~0.3 s before terminal) — dead. (b) The pane is a **rendered** view whose `capture-pane` text
+> no longer contains the answer's source bytes (`## `, `**`, code fences are gone) — dead, and worse
+> than "lossy": it is *not the model's text*. **But there is a third source neither this backlog nor
+> the first spike considered: `claude` fires a `MessageDisplay` hook carrying incremental,
+> byte-faithful `delta`s of the raw reply.** Verified live on a plain interactive TUI spawn (no `-p`),
+> banner `· Claude Max`: 7 fires spread across generation, `concat(deltas) === T` **byte-exactly**
+> (579 == 579), `T.startsWith(S)` true at every step, `## ` / `**` / ```` ```javascript ```` all
+> present in the deltas. Granularity is block-level (~5–7 chunks/answer), not token-level — plenty for
+> SSE. **Build it.**
 >
-> Full evidence, the value re-assessment, and the maintainer's options: **[`streaming-spike.md`](streaming-spike.md)**.
-> The original framing is preserved below for the record.
+> ⚠️ Two corrections to this item as written: the **"~20 s" is wrong** (inferred from an external
+> report, never measured through OCP — the same-turn decomposition puts OCP's own overhead at **~4 s**,
+> n=1), and **streaming moves the first byte, not the last** — so a consumer needing the *complete*
+> answer (the JSON-card case that motivated this) gains **nothing** from it. Build it for
+> progressively-rendering consumers, not as a throughput win.
+>
+> Full evidence + implementer caveats (the hook is `forceSyncExecution` — claude BLOCKS on it):
+> **[`streaming-spike.md`](streaming-spike.md)**. Original framing preserved below.
 
 Today `runTuiTurn` blocks on the transcript until the turn is *finished*. The pane is already
 rendering tokens incrementally the whole time — this harness proves you can observe first token