Draft the diff-line-metadata OSC spec and settle the OSC number

Resolve the OSC number from the 456 placeholder to 1717. There is no central
registry for OSC numbers, so the audit reduces to: pick a high, distinctive
number that no real terminal *acts on* — an unknown OSC is skipped harmlessly,
but a recognized one can fire a visible side-effect (OSC 555 flashes foot, OSC
777 raises a desktop notification), and the metadata flows through real
terminals whenever the pager runs outside a host. Audited the live OSC
allocations of xterm, VTE, kitty, foot, WezTerm, iTerm2, Windows Terminal,
Ghostty, VS Code, ConEmu and urxvt; 1717 collides with none and sits in the
empty 1400-5000 band (only iTerm2's 1337 is nearby).

Write the spec as a standalone draft to circulate to pager developers for
feedback — separate from the internal session notes, with motivation, the v1
wire format, the env handshake, semantics, emit rules (including the per-row
wrapping correction and the side-by-side two-records-per-row case), and the
known v2 candidates (both-numbers-always, the difftastic token-vs-line
mismatch). The prototype code still emits the 456 placeholder; the 456->1717
rename across delta/difftastic/gocui/lazygit and the EMIT_OSC<n>_METADATA env
var is a tracked follow-up, not part of this commit.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

diff-line-metadata-notes.md

@@ -218,17 +218,30 @@ through `Diff.AdjustLineNumber` before opening the editor, exactly as today
   OSC the same way). The metadata flowing through a real terminal must be
   harmless — this is a hard requirement, since pagers run outside lazygit too.
 
-### 3.4 The OSC number
+### 3.4 The OSC number — RESOLVED: `1717`
 
-No central registry, but allocations are rare; pick a **high, distinctive**
-number (the iTerm2 `1337` convention) and **verify against the sources of
-xterm, VTE, kitty, foot, WezTerm, iTerm2, Windows Terminal** before committing.
-`456` is a placeholder. Known-used slots to avoid (from memory, against a
-knowledge cutoff — re-check): `0/1/2` (title/icon), `4`+`104` (palette),
-`7` (cwd), `8` (hyperlinks), `9` (notifications; `9;4` progress), `10–12`+
-`110–119` (colors), `50` (font), `52` (clipboard), `99` (kitty notifications),
-`133` (semantic prompt), `633` (VS Code shell integration), `777` (urxvt),
-`1337` (iTerm2).
+> **RESOLVED (audit done).** The final number is **`1717`**, replacing the `456`
+> placeholder. There is no central registry; the convention is to pick a high,
+> distinctive number and verify no real terminal *acts on* it (an unknown OSC is
+> skipped by conformant terminals, but a *recognized* one can fire a visible
+> side-effect — e.g. `OSC 555` flashes foot, `OSC 777` raises a desktop
+> notification — which is exactly the wire-safety hazard the audit exists to
+> avoid). Audited the live OSC allocations of **xterm, VTE, kitty, foot, WezTerm,
+> iTerm2, Windows Terminal, Ghostty, VS Code, ConEmu, urxvt**; `1717` collides with
+> none and sits in the large empty 1400–5000 band (only iTerm2's `1337` is nearby).
+> The full danger list is in the spec appendix (`diff-line-metadata-osc-spec.md`).
+> **The prototype code (delta/difftastic/gocui/lazygit + the `EMIT_OSC456_METADATA`
+> env var) still uses `456`** — renaming `456`→`1717` across the three repos is a
+> tracked follow-up, not yet done.
+
+Known-used slots to avoid (verified in the audit): `0–3` (title/icon/X11),
+`4/5/6` (palette/special/tab color), `7` (cwd), `8` (hyperlinks), `9`
+(notifications; `9;4` progress, `9;9` cwd), `10–19` (dynamic colors), `21/22`
+(color query / pointer shape), `46/50/51/52` (logfile/font/Emacs/clipboard), `66`
+(text sizing), `99` (kitty notifications), `104–106`+`110–119` (reset colors),
+`133` (semantic prompt), `176` (foot app id), `555` (foot flash), `633` (VS Code),
+`777` (rxvt notify), `1337` (iTerm2), `5522` (kitty clipboard), `30001/30101`
+(kitty color stack).
 
 ### 3.5 Pagers to patch
 
@@ -311,9 +324,11 @@ feedback) and to **inform a from-scratch production plan**.
 > NORMAL (unified, single-column) case — see §8 (#1) and §9 (#2). Side-by-side
 > delta is prototyped (focused-main-view-notes.md §17) and **difftastic is
 > prototyped in both modes (§10)** — so the emitter side now spans the full
-> coverage table. What remains of step 5 is the *deliverables*: finalize and
-> publish the spec (now informed by difftastic's §10.2 model-mismatch finding),
-> and write the production plan. Host *consumption* of side-by-side / difftastic
+> coverage table. What remains of step 5 is the *deliverables*: the **OSC spec
+> draft is written** (`diff-line-metadata-osc-spec.md`, OSC number finalized to
+> `1717` — §3.4 — and it speaks to difftastic's §10.2 model-mismatch finding), ready
+> to circulate to pager developers; what's left is gathering that feedback and
+> writing the production plan. Host *consumption* of side-by-side / difftastic
 > output is still a separate, later step.
 
 Sequence:
@@ -339,8 +354,9 @@ Sequence:
 4. **#2 emitter side** — the delta patch; this is what stress-tests the spec
    against reality, side-by-side being the hard case (§3.5).
 5. **End-to-end → finalize the spec (publish for feedback) → write the
-   production plan.** End-to-end is **done** (§9.4); the spec and production
-   plan are the remaining deliverables.
+   production plan.** End-to-end is **done** (§9.4); the **spec draft is written**
+   (`diff-line-metadata-osc-spec.md`, OSC `1717`). Circulating it for feedback and
+   the production plan are the remaining deliverables.
 
 **Parallel de-risking (any time, doesn't block #1):** confirm by reading delta's
 source that it can produce the fields per region — for `-` lines and in
@@ -482,10 +498,14 @@ layout changes. A **dedicated, purely-additive emitter** (only injects OSC bytes
 never touches styling/width/wrapping) is both safer and cleaner, and its counter
 logic is a near-copy of `linenumbers_and_styles`.
 
-### 9.2 Pinned v1 wire format (placeholder OSC `456`)
+### 9.2 Pinned v1 wire format (final OSC `1717`; prototype code still uses `456`)
+
+> The number was finalized to **`1717`** after the terminal audit (§3.4); the
+> prototype code still emits `456` (rename is a tracked follow-up). The published
+> spec (`diff-line-metadata-osc-spec.md`) uses `1717`.
 
 ```
-ESC ] 456 ; <version> ; <type> ; <new-line> ; <old-line> ; <file> ST
+ESC ] 1717 ; <version> ; <type> ; <new-line> ; <old-line> ; <file> ST
 ```
 
 - `ESC` = `0x1b`; `ST` = `ESC \` (`0x1b 0x5c`) — same framing as delta's OSC-8.
@@ -507,9 +527,11 @@ emits V1 when the advertised list contains `V1`.
 
 ### 9.3 Deferred / known prototype limitations
 
-- **Terminal-source audit of the OSC number is DEFERRED** (§3.4). `456` is a
-  placeholder; before publishing, verify it against xterm/VTE/kitty/foot/WezTerm/
-  iTerm2/Windows Terminal and pick a high, distinctive final number.
+- **Terminal-source audit of the OSC number is DONE** (§3.4). Final number is
+  **`1717`** (audited against xterm/VTE/kitty/foot/WezTerm/iTerm2/Windows Terminal/
+  Ghostty/VS Code/ConEmu/urxvt; danger list in the spec appendix). The prototype
+  code still emits the `456` placeholder — the `456`→`1717` rename across
+  delta/difftastic/gocui/lazygit + the env var is a tracked follow-up.
 - **Wrapped continuation rows** (`Hunk*Wrapped`) get no attachment in the prototype
   — only the primary content row does. Fine for the normal case (gocui's own
   wrapping is handled host-side by the view-line→buffer-line mapping); delta-level