docs: add concise default view design spec

docs/specs/2026-06-21-concise-default-view-design.md

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+# Concise Default View — design spec
+
+## Problem
+
+The index page shows every book grouped by status. As the library grows, this becomes
+too much information at a glance. The user needs a focused default showing only what's
+active right now, with easy paths to browse the rest.
+
+## Design
+
+The index view takes an optional `filter` query parameter. With no filter, it shows a
+concise default: only books in-flight (READING) and the 5 most recently added WANT
+books. All other books are accessible via filter links. With a filter (`all`, `read`,
+`want`), it shows the full matching subset.
+
+## Behaviour
+
+### Default / no filter (concise view)
+
+- **READING group**: all READING books (unchanged from current).
+- **WANT group**: sorted by `:ADDED:` date descending, limited to 5.
+- **READ / ABANDONED**: replaced by summary links — e.g. "Read · 3" as a clickable
+  link to `/index?filter=read`.
+- **Filter nav bar** shown at the top: links to `All books (N)`, `Want list`,
+  `Read (N)`.
+
+### `?filter=all`
+
+Full index, all groups, no truncation — identical to today's default.
+
+### `?filter=read`
+
+Only the READ group. Single group in the listing.
+
+### `?filter=want`
+
+Only the WANT group. Single group in the listing.
+
+### Error handling
+
+- An unrecognised/empty filter value is treated as `nil` (concise default).
+- A WANT book with no `:ADDED:` property sorts before dated entries (treated as
+  oldest).
+- Fewer than 5 WANT books → all are shown, no padding or placeholder rows.
+
+## Implementation
+
+### Model: `spine-index-model`
+
+Signature changes from:
+```
+spine-index-model (books &optional selected-id)
+```
+to:
+```
+spine-index-model (books &optional filter selected-id)
+```
+
+When `filter` is nil, after building the group alist:
+- Sort the `:want` group entries by `added` descending (ISO date string; nils sort
+  first = oldest).
+- Truncate to 5 entries.
+- Replace `:read` and `:abandoned` groups with a `:summary-groups` list. Each
+  entry is a plist matching the existing model convention:
+  ```
+  (:label "read" :count 3 :href "/index?filter=read")
+  ```
+  The template renders these as clickable `.row` links.
+  When `filter` is non-nil, `:summary-groups` is absent.
+When `filter` is non-nil, return only the matching group (or all groups for `"all"`).
+
+### Handler: `defservlet index`
+
+Read the `filter` key from the request query alist. Pass it to `spine-index-model`.
+Pass `current_filter` in the template context for conditional rendering.
+
+### Template: `index.mustache`
+
+- **Filter bar** (above groups): shown when `current_filter` is nil (links to expanded
+  views) or truthy (link back to concise view).
+- **Summary groups**: new `{{#summary_groups}}` block renders a simple `.row` link per
+  suppressed group (READ, ABANDONED).
+- Existing group rendering unchanged — still handles full listing when `filter=all`
+  or single-group views.
+
+### Sorting
+
+Within the WANT group for the concise view: sort by `added` field descending. The
+`added` field is already part of each book's model entry. Nil dates sort before dated
+entries (oldest).
+
+## Out of scope
+
+- Pagination for large WANT lists (5 is a design choice; if the user wants more, the
+  "All books" link is there).
+- Remembering the user's last selected filter across requests (stateless is simpler).
+- Keyboard-driven filter switching (plain links for now).