Implement fit-mode rendering contract with container sizing and pane re-fit

docs/IMPLEMENTATION-PLAN.md

@@ -174,7 +174,7 @@ set to Plex Mono explicitly since it can't read the CSS token.
 
 ---
 
-## M2 · Editor robustness
+## M2 · Editor robustness ✅ (done)
 
 **Goal:** the editor becomes trustworthy — draft vs published, schema-aware
 assistance, and the fit-mode rendering contract.
@@ -185,7 +185,9 @@ assistance, and the fit-mode rendering contract.
   Vega-Lite `"container"`), recursing into layered/concat/child specs (spec §04
   Rendering Contract, step 2).
 - `vega-lite-schema.ts` — provide the Vega-Lite JSON schema for Monaco's
-  validation/autocomplete (mine vega-editor for sourcing/versioning the schema).
+  validation/autocomplete. _Delivered early in M1.5 as
+  `infrastructure/monaco-schema.ts` (bundled schema, offline, `markdownDescription`
+  hover docs); no further work needed in M2._
 
 **App**
 
@@ -210,6 +212,30 @@ assistance, and the fit-mode rendering contract.
 - Invalid spec shows inline error; autocomplete suggests Vega-Lite properties.
 - Each fit mode resizes the chart as specified; choice survives reload.
 
+**Verified:** `typecheck` + `test` (83 passing — `rendering` fit-mode incl.
+nested layer/concat/facet specs, `SnippetStore` draft/publish/revert/editorView,
+`settings-store` `previewFitMode` round-trip) + `build` (PWA, 41 precache
+entries) + `eslint` clean. Implementation notes: editing now writes the
+**draft** only (`commitDraft` no longer touches `spec`); `publish`/`revert` live
+in `SnippetStore`, with a `bufferEpoch` counter so programmatic buffer reloads
+(select/create/revert) refresh Monaco without fighting the cursor mid-typing.
+The Draft/Published view is a store-level `editorView`; the published view is
+read-only and the preview renders whichever version is shown (`selectShownText`).
+The editor (§03E) and preview (§04) share one render error via a small
+`PreviewStore`. `previewFitMode` was pulled into `AppStore` + the settings
+adapter, hydrated/persisted by a new `orchestration/preferences.ts` mirroring the
+theme slice. Publish/Revert **success toasts** stay deferred to M6 (TODO
+breadcrumbs at the call sites), matching the existing delete-toast convention.
+
+Fit-mode rendering needed a layout fix: vega-embed brands the embed host with its
+own `.vega-embed { display: inline-block }` (injected at runtime, wins the
+cascade), which shrink-wrapped the host so `width: "container"` collapsed (Height
+survived only via the old `min-height: 100%`). Fix: embed into a static-class
+inner host (React never reconciles its className, so Vega's runtime classes
+survive) inside a React-owned frame that carries the fit-sizing class via
+two-class selectors that out-specify `.vega-embed`. All four fit modes
+user-verified in the running app.
+
 ---
 
 ## M3 · Datasets
@@ -329,8 +355,12 @@ the reference.
 
 **Goal:** the workspace feels finished and meets §10.
 
-- **Panes:** drag-resize handles with min widths; per-pane show/hide toggle strip;
-  widths + visibility persist (§01A, §09D).
+- **Panes:** ~~drag-resize handles with min widths; widths persist~~ ✅ **pulled
+  forward after M2** (coupled to the preview's container sizing — see
+  [arch 05 §8](architecture/05-rendering-theming-preview.md)). Side panes carry
+  remembered widths, the editor flexes between them, widths persist to
+  `astrolabe:ux-prefs`. **Remaining:** per-pane show/hide **toggle strip** +
+  visibility persist + proportional redistribution on hide (§01A, §09D).
 - **Routing:** URL hash view-state (`#snippet-<id>`, `#datasets/...`) with Back/Forward;
   restore on load (§01E). _(see [Architecture 04 · Routing & Events](architecture/04-routing-and-events.md))_
 - **Shortcuts:** Cmd/Ctrl+Shift+N / +K / +S / +, / Esc via a single key router