feat(ui): History & Versions diff — the prototype (#236)

[angela-helios]

Closes #236. This is the prototype you asked for on #246 ("implement a prototype and we can evaluate from there"), built to the decision recorded in docs/history-diff-rendering.md.

What it does

Compares two versions of a resource from _history, rendering the diff server-side in the two layers the decision doc settled on — no client-side diff library, works as a plain page with JS off degrading to the base view.

Semantic layer (json-patch) — the field-level change list, the exact inverse of the RFC 6902 PATCH the server already applies:

REPLACE  /name/0/family   Smith → Smythe
REPLACE  /name/0/given/0  John  → Johanna
ADD      /name/1          {"family":"Smith","given":["John"],"use":"nickname"}
ADD      /telecom/1       {"system":"email","value":"j@x.org"}

json-patch was already in the workspace lock (a default dep of rest/persistence), so it adds zero new transitive weight.

Textual layer (similar, inline) — paired lines, +/- gutters, per-side line numbers, and word-level highlighting of the run that changed within a line. Selecting v1 → v3 shows it well: "Jon" → "John" highlights only the inserted letter.

Following the decision doc

• Metadata noise floor: meta.versionId/lastUpdated move every version → filtered by default, behind a "show metadata" toggle that says how many ops it hid.
• Version selection: adjacent (n vs n-1) by default; two selects, htmx-swapped.
• Where computed: server-side, in the helios-ui handler. No new FHIR-facing endpoint — the browser reads versions from the ordinary _history/vread API and posts them to /ui/history/diff.
• Deleted versions: a banner, not a diff against a tombstone. Identical versions say so.

Following Brett's design

The Figma "History & Versions" frame: the Instance / Type feed / System feed tabs, the version rail (newest first, each tagged create / update / patch), and the diff panel. The nav's History & Versions entry is now a real link.

Verified in a real browser

Driven end to end against a Patient with three real versions (create → update → update). One detail only surfaced by driving it: this server carries the version id and timestamp in the history Bundle's entry.response etag/lastModified, not in resource.meta — so the rail reads there, with resource.meta as a fallback for servers that stamp it onto the resource.

Tests

17 unit tests over the diff layer — including the exact array-churn scenario the decision doc's spike characterizes — plus 6 HTTP round-trip tests, locale parity across en/es/de. fmt + clippy clean.

Deliberately deferred (matches the decision doc)

• Array-churn move detection: a front-inserted name[] still cascades across name/0..2 instead of a single add /name/0. The doc explicitly accepts this for v1; move detection is a presentation post-process that can be added later without changing the architecture, and the textual layer already shows the insert legibly.
• The Type feed / System feed tabs are present (Brett's design) but not yet wired to /{type}/_history and /_history.
• The metadata toggle does nothing visible against this server, because it does not stamp meta onto the resource body — but the filter is implemented and unit-tested against resources that carry meta (which most real resources do).