docs(lane): two precision fixes, and make 1b's block a real checkpoint

The block said the platform decision "replaces a `cfg!` read". It does
not: production still reads `cfg!(target_os = "linux")` at
pmacs-gpu/src/main.rs:3450. What became injectable is the SELECTION
decision taken after that target fact --- `is_linux` comes from `cfg!`
in a non-test build and from an override otherwise, and
`paste_source_for(is_linux)` is the part a row can drive both ways.

And "L1-L8 still owed" hid implemented state that recovery needs. The
manual authority latch, its wrap/replacement clearing, and R4/R5's two
separate residual resets have all LANDED; what is owed is their
witnesses. The block now splits "landed but not yet witnessed" from
"owed outright" rather than letting one phrase imply neither exists.

It also names the authoritative ref and the exact recovery command,
which is what this file is for and what the block did not have.

docs/active-work.md

@@ -303,7 +303,18 @@ waits for a signal that is not coming.
 
 ## GUI arc Stage 1b — pointer and scroll — IMPLEMENTING
 
-**Branch `gui-stage1b-pointer-scroll`, rebased onto `0ec13b3`.** Framing
+**Branch `gui-stage1b-pointer-scroll`, rebased onto `0ec13b3`.** The
+**remote ref `githubsucks/gui-stage1b-pointer-scroll` is
+authoritative**; recover with
+`git fetch githubsucks && git checkout -B gui-stage1b-pointer-scroll githubsucks/gui-stage1b-pointer-scroll`.
+
+**The rebase rewrote history**, so that ref was force-pushed. The seven
+pre-rebase framing commits are preserved on
+**`githubsucks/gui-stage1b-prerebase-backup`** (`5c83b15`) — pushed
+before the rewrite, not after, so nothing depended on a local-only ref
+surviving.
+
+Framing
 `docs/gui-stage1-input-framing.md`, **revision 20** — §2a re-measured at
 this base, the panel-replay prerequisite recorded as DISCHARGED by #243,
 and the both-axis effect witness still owed.
@@ -313,10 +324,22 @@ producer), B2's daemon-side horizontal panel leg, B3/B7's shared
 `scroll_window_columns` with its saturated bound and wrap pin, and B4's
 middle-click PRIMARY paste.
 
-**Still owed:** B5, B6's routing row, the L1–L8 lifetime rows, step 3's
-fractional both-axis panel witness, and **B1's disposal half** — a
-residual keyed to a surface that goes away must go with it, and this
-frontend does not yet track "that buffer is gone".
+**Landed but NOT yet witnessed** — recovery needs this split, because
+"L1–L8 owed" reads as though none of the mechanism exists:
+
+- the **manual horizontal authority latch** (`manual_left_authority`),
+  armed only by an *effective* move (clause 2);
+- **wrap and buffer-replacement clearing** of that latch and the origin
+  (clause 5), on both the GPU's `scroll_by_columns` and the buffer
+  replacement path;
+- **R4 and R5's residual resets**, as two separate clears beside
+  `code_scroll_left` so omitting either is individually visible.
+
+**Owed outright:** B5, B6's routing row, the **L1–L8 rows** for the
+latch above, step 3's fractional both-axis panel witness, R4/R5's own
+replacement witnesses, and **B1's disposal half** — a residual keyed to
+a surface that goes away must go with it, and this frontend does not yet
+track "that buffer is gone".
 
 ### Two gate gaps this lane found, neither fixed here
 
@@ -341,9 +364,13 @@ the next lane does not rediscover them at review.
 appears exactly once in `ci.yml`, in the Ubuntu-only `gpu-render` job;
 the macOS matrix tests the workspace default member only. B4's
 off-Linux contract would therefore have been asserted nowhere that
-executes, so **the platform decision is a parameter rather than a
-`cfg!` read** and both outcomes are driven on this host. Adding a macOS
-`pmacs-gpu` leg is the real fix and is not this lane's.
+executes. **Production still reads `cfg!(target_os = "linux")`**
+(`pmacs-gpu/src/main.rs:3450`); what became injectable is the
+**selection decision taken after** that target fact — `is_linux` is
+obtained from `cfg!` in a non-test build and from a test override
+otherwise, and `paste_source_for(is_linux)` is the part a row can drive
+both ways. Adding a macOS `pmacs-gpu` leg is the real fix and is not
+this lane's.
 
 ## CRDT identity-replace undo — MERGED as #246 (`78346de`)