dev: scripts/bite — machine-checkable bite-verification

A trap-guarded one-file swap over read-only git show: run the named
tests against an older version of one file and exit 0 iff they FAIL
there (the fix bites); exit 1 flags a vacuous fix. The file is
restored on every exit path including interrupts; the ./ pathspec
prefix makes it work from any directory inside the repo. Never
touches git stash — the stash namespace is repo-global, shared
across worktrees and with humans (a #111-round mishap popped a
PR-#17-era user stash; hence the tool).

Dogfooded three ways against the merged #111 history: the round-1
fix commit bites (exit 0), the same tests against HEAD read as
vacuous (exit 1), and a docs/-relative invocation restores cleanly.

Handoff: section 1 snapshot (editops #111 merged, worktree folded),
section 5 lessons (repo-global stash; worktree-per-lane for
parallel work).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018vF4gQVozBWi38y1SJiGfQ

docs/agent-handoff.md

@@ -1,7 +1,7 @@
 # Agent handoff — cross-machine continuity
 
 **Last updated: 2026-07-12, on the laptop, by the editops session
-(merging the auto-pairing #110 post-merge sync).** This file is the
+(post-#111 merge; scripts/bite micro-PR).** This file is the
 bridge between development machines. If you are an agent reading
 this on a fresh clone: this document plus the `docs/*-framing.md`
 files ARE your memory. Read this fully before taking on work, seed
@@ -11,22 +11,23 @@ reads it the way you just did.
 
 ## 1. Where the project stands (2026-07-12)
 
-- `main` @ `4174f3e` (auto-pairing #110 merged), protocol **v15**
+- `main` @ `f0a05c5` (editops #111 merged), protocol **v15**
   (`SUPPORTED=[6..15]`).
-- **Editing-conveniences pack (editops) in flight: PR #111** — the
-  Lua parallel lane, branch `editops`. Framing
-  `docs/editing-conveniences-framing.md` at revision 6 (three
-  pre-branch rounds, one adopted post-approval hardening, and PR
-  round 1: full UTF-8 scalar validation, per-word capitalize with
-  the `_`-constituent deviation named, trim-on-save dual-channel
-  error reporting). goto-line, case ops, transpose, zap-to-char
-  (kill-chain member with an origin guard and killring's new
-  pending-prompt marker), line move/duplicate/join, region
-  sort/reverse/dedupe, delete-trailing-whitespace + opt-in
-  trim-on-save. killring.lua gains `kill_range` /
-  `break_chain([fid])` / the marker lifecycle. On the laptop this
-  branch lives in a git worktree at `../pmacs-editops`; fold it
-  back after merge.
+- **Editing-conveniences pack (editops, #111) landed** — the Lua
+  parallel lane. Framing `docs/editing-conveniences-framing.md` at
+  revision 6 (three pre-branch rounds, one adopted post-approval
+  hardening, one PR round: full UTF-8 scalar validation, per-word
+  capitalize with the `_`-constituent deviation named, trim-on-save
+  dual-channel error reporting). Ships goto-line, case ops,
+  transpose, zap-to-char (kill-chain member with an origin guard
+  and killring's pending-prompt marker), line move/duplicate/join,
+  region sort/reverse/dedupe, delete-trailing-whitespace + opt-in
+  `pmacs.editops.trim_on_save`. New substrate other code can use:
+  `pmacs.killring.kill_range` / `break_chain([fid])` /
+  `arm_kill_prompt`+`commit_kill_prompt` (the marker lifecycle),
+  and the origin-guard pattern for chain-sensitive minibuffer
+  commands. The development worktree at `../pmacs-editops` has been
+  folded back.
 - **Auto-pairing (#110) landed — Arc 2 is COMPLETE.** Framing
   `docs/auto-pairing-framing.md` at revision 6 (two pre-branch rounds
   + three PR rounds). Shape that shipped: the nine built-in pair
@@ -168,7 +169,18 @@ New wire surface ⇒ bump + both-frontends support + acceptance.
 - **The checkout may be shared with the user.** Check `git status` for
   foreign uncommitted work before any stash/checkout/branch surgery;
   never assume dirty files are yours. (Their uncommitted fix was nearly
-  orphaned once.)
+  orphaned once.) A clean status goes stale within minutes when two
+  lanes are active — for parallel work, `git worktree add` a sibling
+  directory off main instead of switching the shared checkout.
+- **Never `git stash` in this repo.** The stash namespace is
+  REPO-GLOBAL — one list shared across every worktree and with the
+  user; a scripted push/pop can pop a human's years-old stash into
+  your tree (happened during #111: a failed `stash push` chained
+  into `stash pop`, which grabbed the user's PR-#17-era entry). For
+  run-tests-against-an-old-version swaps, use `scripts/bite` — a
+  trap-guarded one-file swap over read-only `git show`, with an
+  inverted verdict (exit 0 iff the tests FAIL against the old
+  version), making bite-verification machine-checkable.
 - **Stacked PRs**: retarget the child to main BEFORE merging the
   parent — GitHub auto-closes a PR whose base branch is deleted and
   cannot reopen it (#104 → re-opened as #105).

scripts/bite

@@ -0,0 +1,60 @@
+#!/bin/sh
+# scripts/bite --- prove a fix's tests BITE: run them against an older
+# version of ONE file and succeed only if they FAIL there.
+#
+#   scripts/bite <ref> <path> [cargo-test-args...]
+#
+# Example (the PR #111 round-1 shape):
+#   scripts/bite HEAD~1 builtin/runtime/editops.lua \
+#       --test editops_acceptance -- capitalize trim_on_save_unexpected
+#
+# Exit status: 0 when the named tests fail against <ref>'s version of
+# <path> (the fix bites), 1 when they still pass (vacuous), 2 on
+# usage/setup errors. The working-tree file is restored on every exit
+# path, including interrupts.
+#
+# Caveat: a COMPILE error of the old tree also counts as "fails" —
+# correct, but weaker evidence than a clean assertion failure; eyeball
+# the output when the swapped file is Rust rather than Lua.
+#
+# Why this exists: bite-verification is step 4 of the working method,
+# and the obvious shortcut — git stash — is a trap here. The stash
+# namespace is REPO-GLOBAL: shared across every worktree and with
+# humans, so a scripted push/pop can collide with (or pop!) someone
+# else's stashed work. This helper never touches git state beyond a
+# read-only `git show`.
+
+set -eu
+
+if [ "$#" -lt 2 ]; then
+    echo "usage: scripts/bite <ref> <path> [cargo-test-args...]" >&2
+    exit 2
+fi
+
+ref=$1
+path=$2
+shift 2
+
+if [ ! -f "$path" ]; then
+    echo "bite: no such file: $path" >&2
+    exit 2
+fi
+
+saved=$(mktemp "${TMPDIR:-/tmp}/bite.XXXXXX")
+cp -- "$path" "$saved"
+restore() {
+    cp -- "$saved" "$path"
+    rm -f -- "$saved"
+}
+trap restore EXIT INT TERM
+
+# The `./` prefix makes the pathspec cwd-relative for git-show, so the
+# script works from any directory inside the repo.
+git show "$ref:./$path" > "$path"
+
+if cargo test "$@"; then
+    echo "bite: VACUOUS --- tests still pass against $ref:$path" >&2
+    exit 1
+fi
+
+echo "bite: OK --- tests fail against $ref:$path (the fix bites)"