spec(0062): staged delivery (§7.1) + companion feasibility spike plan

§7.1 now records the 4 de-risking rungs (mockups → throwaway feasibility
spike → vertical slice → full build), riskiest-API-assumption-first, so any
forced design change lands on throwaway code. Adds coauthoring-feasibility-
spike.md: 5 experiments (E1 editable proposals+Keep/Undo, E2 baseline router,
E3 preview authorship, E4 comment loop, E5 discoverability) with PASS/FAIL
criteria; closes open Q2/Q4/Q7 before any plan is written.

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

specs/coauthoring-feasibility-spike.md

@@ -0,0 +1,114 @@
+---
+status: draft
+---
+# Feasibility Spike Plan — Coediting Markdown with a Machine (native surfaces)
+
+| | |
+| --- | --- |
+| **Companion to** | `coauthoring-native-surfaces.md` (Solution Design) — §7.1 rung 2 |
+| **Type** | Throwaway discovery **prototype** (handbook §5.3) — code is disposable |
+| **Goal** | Prove the API-dependent design assumptions on the **real** VS Code surfaces, cheaply, **before** the full build — so post-ship iteration is polish, not design |
+| **Non-goal** | Production code, real Claude/SDK wiring (except E4), persistence durability, polish, breadth |
+| **Status** | `draft` |
+
+---
+
+## Why this exists
+
+Static mockups validated the *look* and *state coverage*; they prove **nothing**
+about whether VS Code's native surfaces actually behave as the spec assumes. Twice
+already, an unexamined assumption would have forced a redesign (the diff can't
+recolor per author; the Comments API can't render in the preview). The remaining
+high-risk assumptions can only be settled by **building on the real API**. This spike
+does exactly that and nothing more: each experiment returns a **PASS / PASS-with-
+fallback / FAIL**, and any FAIL becomes a spec delta folded back into
+`coauthoring-native-surfaces.md` before a plan is written.
+
+## Shape & guardrails
+
+- **One throwaway VS Code extension** (`vscode-cowriting-prototype`), run in the
+  Extension Development Host. **Delete it when the spike ends** — none of this code
+  graduates.
+- **The machine is stubbed** for E1–E3, E5 (a function returning canned `before/after`
+  edits) so we test *VS Code feasibility and feel*, not the SDK. E4 may use a minimal
+  real call only if cheap.
+- **Time-box: ~2–3 focused days.** If an experiment can't be made to PASS or
+  PASS-with-fallback inside its slice, that is itself the finding — record it and move
+  on.
+- **No persistence/durability work**, no anchoring ladder, no cross-rung format — those
+  are not API-risk; defer to the build.
+
+## Experiments (riskiest first)
+
+### E1 — Editable proposed side + Keep/Undo *(highest risk)*
+- **Assumption (INV-12, Q2, Q7):** the machine's proposal can be reviewed in a
+  multi-diff / diff editor whose **proposed side is writable**, the writer can tweak it
+  before keeping, and **per-change Keep/Undo** works.
+- **Build:** open a diff between a read-only baseline (`cowriting-baseline:` virtual
+  doc) and a **writable** proposal doc (untitled/in-memory) seeded with a canned edit;
+  attach Keep/Reject actions (try the multi-diff overlay; fall back to per-hunk
+  **CodeLens**). Edit the proposal side; on Keep, apply to the "live" doc and attribute
+  machine-vs-tweak by diffing final ↔ canned ↔ baseline.
+- **PASS:** proposed side is editable; Keep applies the *tweaked* text; attribution
+  splits correctly; live doc untouched until Keep.
+- **Decides:** Q2 (stable ceiling vs CodeLens fallback), Q7 (Keep/Undo after free
+  cross-hunk edits). **If FAIL:** the accept/reject surface design changes — know now.
+
+### E2 — Baseline router (git HEAD + snapshot)
+- **Assumption (INV-7):** a `QuickDiffProvider` against a `cowriting-baseline:` doc
+  shows gutter bars + "Open Changes"; the original resolves from **git `HEAD`** when
+  tracked and from a **snapshot** otherwise; the diff goes **clean on commit /
+  Mark-Reviewed**.
+- **Build:** register the provider; for a repo file read `HEAD` via the Git extension
+  API and recompute on `repository.state.onDidChange`; for a `~/Downloads` / untitled
+  file use a stored/in-memory snapshot. Commit (git) and "Mark Changes as Reviewed"
+  (snapshot) → confirm the diff clears.
+- **PASS:** both modes render the expected diff; both resets clear it; works for an
+  untitled doc.
+
+### E3 — Preview authorship via markdown-it
+- **Assumption (PUC-3, D11):** a contributed `markdown.markdownItPlugins` /
+  `extendMarkdownIt` plugin injects per-span **authorship colors + deletions** into the
+  **built-in** Markdown preview, toggled by a setting/command, with no custom webview.
+- **Build:** contribute the plugin; feed canned attributions/changes; render
+  green/blue + strikethrough; toggle `cowriting.annotations` on/off.
+- **PASS:** marks render inside the native preview; toggle works; deletions show;
+  scroll-sync intact. **If FAIL:** authorship-surface design changes.
+
+### E4 — Comment → reply → offer → proposal loop
+- **Assumption (PUC-8, D10):** a Comments-API thread on a coedited doc can fire a turn,
+  post a **machine reply `Comment`**, and convert an accepted **offer** into a pending
+  proposal (E1's surface). Also: a **preview selection → native thread on source** via
+  the markdown-it token `.map` (D15).
+- **Build:** create a thread on selection (and from a preview selection mapped to
+  source); on add, run a stubbed-or-minimal turn; post a reply comment with an
+  "Make this edit" action that opens an E1 proposal.
+- **PASS:** reply appears in-thread; offer→proposal works; preview-initiated thread
+  lands on the correct source range. *(Feel/latency notes captured.)*
+
+### E5 — Discoverability (title bar + status bar)
+- **Assumption (INV-13):** the editor **title-bar** actions (`editor/title` menus) and
+  a **`StatusBarItem`** (`✦ Coediting · N changes`, click → review) are visible and
+  obvious without the Source Control pane; everything gates on `cowriting.isCoediting`.
+- **Build:** contribute the menus + status-bar item gated on the context key; enter/exit
+  coediting flips them.
+- **PASS:** a first-time (non-git) user can find "review changes" without opening Source
+  Control. *(Cheapest experiment — fold into the others.)*
+
+## Output
+
+A short **findings report** (PASS / PASS-with-fallback / FAIL per experiment, with the
+feel notes for E1/E4) and, for any non-PASS, a concrete **spec delta** applied to
+`coauthoring-native-surfaces.md` — explicitly resolving open questions **Q2, Q4, Q7**.
+Then the code is deleted and delivery moves to rung 3 (vertical slice).
+
+## Mapping to the design
+
+| Experiment | Validates | Closes |
+| --- | --- | --- |
+| E1 | INV-12 · editable proposals + Keep/Undo | Q2, Q7 |
+| E2 | INV-7 · baseline router (HEAD + snapshot) | — |
+| E3 | PUC-3 / D11 · preview authorship | — |
+| E4 | PUC-8 / D10 / D15 · comment loop + preview-initiated thread | — |
+| E5 | INV-13 · title-bar / status-bar discovery | — |
+| (any FAIL needing a non-native surface) | INV-3 escape hatch | Q4 |