docs(rtm): trace the grouped-view contained-card layout (#827)

Refs #827

.specify/rtm.md

@@ -209,10 +209,10 @@
 | REQ-011 | ≤320px: control overflow-free + tap ≥44px, each abbreviation carries its full word as aria-label | #827 | timeline-grouping-modes | `frontend/src/lib/timeline/GroupingControl.svelte` | `GroupingControl.svelte.spec.ts#exposes each segment full word as an aria-label`, `e2e/zeitstrahl-grouping.spec.ts#the control stays overflow-free and operable at 320px` | Done |
 | REQ-012 | new grouping/bucket Paraglide keys in de/en/es; no collision with existing timeline*_ keys | #827 | timeline-grouping-modes | `frontend/messages/{de,en,es}.json` | `messages.spec.ts#zeitstrahl grouping + bucket keys are present in all locales (#827 REQ-012)`, `messages.spec.ts#de, en, and es have identical key sets` | Done |
 | REQ-013 | failed timeline fetch → existing localized error via getErrorMessage; grouping has no independent failure mode | #827 | timeline-grouping-modes | `frontend/src/routes/zeitstrahl/+page.server.ts` (#779, unchanged) | `zeitstrahl/page.server` error path (#779 — getErrorMessage(extractErrorCode)) | Done |
-| REQ-014 | Ereignis event-clustered letter renders as the `.lcard.ev` variant, **nested directly under its event pill** with no duplicate title (redesign #847) | #827 | timeline-grouping-modes | `frontend/src/lib/timeline/LetterCard.svelte`, `frontend/src/lib/timeline/LetterBucket.svelte`, `frontend/src/lib/timeline/YearBand.svelte` | `LetterCard.svelte.spec.ts#carries the .lcard.ev class in the event variant`, `LetterBucket.svelte.spec.ts#renders its letters as .lcard.ev event cards`, `YearBand.svelte.spec.ts#nests an event cluster under its pill in the same year without repeating the title` | Done |
+| REQ-014 | Ereignis event-clustered letters live inside a **contained card whose header is the same-year curated event** (glyph, title, date, provenance, edit pencil) — the title reads once, no separate floating pill; letters render as the compact `.lcard.ev` variant, first 5 + show-more (redesign #847 → #827 grouped-card layout) | #827 | timeline-grouping-modes | `frontend/src/lib/timeline/LetterCard.svelte`, `frontend/src/lib/timeline/LetterBucket.svelte`, `frontend/src/lib/timeline/YearBand.svelte` | `LetterCard.svelte.spec.ts#carries the .lcard.ev class in the event variant`, `LetterBucket.svelte.spec.ts#renders the curated event as the card header when given an `event` (no separate pill)`, `LetterBucket.svelte.spec.ts#shows no edit affordance in the header when canWrite is false`, `YearBand.svelte.spec.ts#renders a same-year curated event as one card header, with no separate pill and no duplicate title` | Done |
 | REQ-015 | Thema bucket-header chip tinted via rootTagColor `var(--c-tag-*)`, neutral fallback when null/unknown; **label kept in a fixed ink for ≥4.5:1 contrast** (redesign #847) | #827 | timeline-grouping-modes | `frontend/src/lib/timeline/BucketHeaderChip.svelte` | `BucketHeaderChip.svelte.spec.ts#tints the chip with var(--c-tag-{token})`, `#renders a neutral chip with no --c-tag- binding when colour is null`, `#falls back to neutral for an unknown colour token`, `#paints the label in a fixed ink colour, never the saturated tag token` | Done |
 | REQ-016 | header meta-line grouping segment tracks the active mode (date/event/thema keys) | #827 | timeline-grouping-modes | `frontend/src/routes/zeitstrahl/+page.svelte` | `zeitstrahl/page.svelte.spec.ts#updates the meta-line grouping label when a mode is chosen` | Done |
 | REQ-017 | Thema: per-letter TagChip suppressed inside its own bucket; still shown in Datum/Ereignis | #827 | timeline-grouping-modes | `frontend/src/lib/timeline/LetterCard.svelte`, `frontend/src/lib/timeline/LetterBucket.svelte` | `LetterCard.svelte.spec.ts#suppresses the per-letter tag chip when asked`, `#still shows the per-letter tag chip when not suppressed`, `LetterBucket.svelte.spec.ts#suppresses the per-letter tag chip inside its own root-tag bucket` | Done |
 | REQ-018 | Letters layer off → grouping control disabled (kept in place), mode retained | #827 | timeline-grouping-modes | `frontend/src/routes/zeitstrahl/+page.svelte`, `frontend/src/lib/timeline/GroupingControl.svelte`, `frontend/src/lib/timeline/timelineGrouping.ts#hasLooseLetters` | `zeitstrahl/page.svelte.spec.ts#disables the grouping control when the Letters layer is off`, `GroupingControl.svelte.spec.ts#retains the active mode while disabled`, `timelineGrouping.spec.ts#hasLooseLetters` | Done |
 | REQ-019 | Ereignis: letter whose only linking event was filtered off → "Weitere Briefe" (never re-introduced) | #827 | timeline-grouping-modes | `frontend/src/lib/timeline/timelineGrouping.ts#buildEventLookup`, `frontend/src/lib/timeline/TimelineView.svelte` | `timelineGrouping.spec.ts#drops a letter whose linked event is absent from the lookup into fallback` | Done |
-| REQ-020 | Grouped buckets are bound by a colour rail and carry compact cards; a bucket over `BUCKET_DENSE_THRESHOLD` (6) collapses to the month-density `YearLetterStrip` instead of flooding the timeline (redesign #847) | #827 | timeline-grouping-modes | `frontend/src/lib/timeline/LetterBucket.svelte`, `frontend/src/lib/timeline/timelineGrouping.ts#isBucketDense`, `frontend/src/lib/timeline/LetterCard.svelte` | `LetterBucket.svelte.spec.ts#collapses an oversized bucket to the density strip instead of flooding cards`, `#binds a tag bucket together with a coloured left rail from its token`, `#renders compact cards for a small bucket`, `LetterCard.svelte.spec.ts#renders the compact variant on a single tighter row` | Done |
+| REQ-020 | Grouped clusters are **contained colour-railed cards** (bordered, rounded, surface) carrying compact cards; a cluster shows the first `CLUSTER_PREVIEW` (5) letters behind a show-more toggle, and the leftover bin is a **collapsed count-only drawer** revealed on demand — the month-density `YearLetterStrip` is no longer used in grouped mode (still used in Datum dense years) (redesign #847 → #827 grouped-card layout) | #827 | timeline-grouping-modes | `frontend/src/lib/timeline/LetterBucket.svelte`, `frontend/src/lib/timeline/timelineGrouping.ts#CLUSTER_PREVIEW`, `frontend/src/lib/timeline/LetterCard.svelte` | `LetterBucket.svelte.spec.ts#renders the cluster as a contained card (bordered, rounded, surface)`, `#binds a tag bucket together with a coloured left rail from its token`, `#shows only the first 5 letters with a show-more toggle when the cluster is larger`, `#expands to all letters and collapses back on toggle`, `#renders collapsed — count + reveal, no letter cards — until opened`, `#reveals the first 5 letters when opened`, `LetterCard.svelte.spec.ts#renders the compact variant on a single tighter row` | Done |