feat(#20): admin (bifrost inspection + admin-events stream) onto the wt adapter (slice-6)

Slice-6 of the worldtree-sdk cutover: migrate the two admin routes off the
hand-rolled paths onto the `ratatoskr.wt` adapter over `client.admin.*`, and delete
the retired code. Both are web-only (the coverage-map's `tui.py` rows were stale —
corrected to `web/server.py`).

Adapter (`wt.py`): `get_session_bifrost` → `client.admin.sessions.bifrost` (open-world
dict verbatim, any error → SessionApiFailed default); `stream_admin_events` →
`client.admin.stream_events`, re-wrapping the SDK's `AdminEvent` → ratatoskr's at the
boundary.

Decisions (contract § slice-6 notes):
- Admin auth moves from a per-call `Authorization` header override to the client's
  `admin_auth` (`_wt_client(admin_key=…)`, extended this slice) — the SDK's admin.*
  routes use the provider, not a header.
- `AdminEvent` re-wrap (chosen over yield-through): the SDK's `admin_id`(nan)/None-able
  `type`/`data` diverge from ratatoskr's `id`/`type`/`data` that the web filter reads;
  re-wrapping (nan→0, None→""/{}) degrades the open-world None/nan ONCE at the adapter
  and keeps the web endpoint + `_admin_event_matches_web` + the `AdminEvent` domain type
  unchanged (preserves the web surface). Rejected: yield SDK events + rewire the web
  filter (heavier churn, scattered hardening).
- Admin-stream error map: a NON-200 open raises `ApiError("admin_stream_failed")`
  (NOT `ConnectFailed`) → SseConnectFailed; `ConnectionDropped` (connect-time OR
  mid-stream/resumable-EOF) → SseConnectionDropped. The web integration test caught the
  ApiError-not-ConnectFailed gotcha the unit fake couldn't.

Web (`web/server.py`): both admin endpoints build the wt client with admin_key and call
`wt.*`; the bifrost endpoint gains ConnectFailed→502 handling (cutover foot-gun); the
admin-events endpoint closes the injected transport (INV-CUT-1), never the wt client.

Deleted the hand-rolled `sessions.get_session_bifrost` + `sse_client.stream_admin_events`
(+ orphaned httpx/httpx_sse/json/AsyncIterator imports); the ratatoskr `AdminEvent`
dataclass stays in `sse_client.py` (re-wrap target, imported by wt + web) until slice-7.
Retired `test_sse_client.py` entirely (its last test was the admin stream) and the
`test_sessions.py` `TestGetSessionBifrost`; added the slice-6 adapter tests.

LIVE SMOKE (:8081, readonly-admin key) — INV-CUT-5 / DEC-4 cleared: the web bifrost
endpoint returned an admin-authed clean 404 envelope (auth + route + mapping proven);
a real `session.created` admin event (id=32) re-wrapped cleanly on live wire (driven by
a session-create, throwaway session cleaned up).

Suite 490 green; ruff clean; mypy net-improved on web/server.py (16→12 pre-existing, no
new). Patch bump 0.21.18 → 0.21.19 (the cutover MINOR is deferred to slice-7, DEC-6).

docs/contracts/worldtree_sdk_cutover.contract.md

@@ -295,6 +295,46 @@ re-anchor its coverage-map rows.
   web-server caller — only the `--whoami` and `--characters` CLI one-shot probes. The
   web surface is untouched this slice.
 
+### Slice-6 notes (Admin: bifrost inspection + admin-events stream, decided at TDD)
+
+- **Admin auth moves from a per-call header override to the client's `admin_auth`.**
+  The SDK's `admin.*` methods authenticate with the client's `admin_auth` provider
+  (set via `build_client(admin_key=...)`), NOT a per-request `Authorization` header. So
+  the two web admin endpoints build their wt client WITH `admin_key` (`_wt_client(client,
+  admin_key=...)`, extended this slice); the hand-rolled per-call `admin_key=` +
+  header-override is retired. The web already guards `if not admin_key: 400` before the
+  call, so the SDK's pre-HTTP `ConfigurationError` (missing admin_auth, W-5) is
+  unreachable from the web surface. **CLI has no admin caller** — both routes are
+  web-only (the coverage-map's `tui.py` rows were stale; corrected to `web/server.py`).
+- **`get_session_bifrost` — no new § Error map row.** `client.admin.sessions.bifrost`
+  returns the open-world `BifrostInspection` dict verbatim; any `ApiError` (notably 403
+  `auth_scope_denied`, 404 `session_not_bifrost_bound`) → the `SessionApiFailed` default
+  — exact parity with the retired path (which mapped every non-200 → `SessionApiFailed`).
+- **`stream_admin_events` re-wraps the SDK's `AdminEvent` → ratatoskr's `AdminEvent`
+  (chosen over yield-through).** The SDK's `AdminEvent` diverges from ratatoskr's:
+  `admin_id: int|float` (`nan` for an id-less envelope) vs ratatoskr's `id: int` (0
+  default), and the SDK's `type`/`data` are None-able where ratatoskr's are a dotted-str
+  / a `{}`-default dict. The web filter + SSE formatter read `ev.id`/`ev.type`/`ev.data`.
+  The adapter re-wraps at the boundary — `id = admin_id if int else 0` (nan→0),
+  `type = type or ""` (None→"" so `.startswith` never crashes), `data = data or {}` —
+  degrading the SDK's open-world None/nan ONCE at the adapter, keeping the web endpoint +
+  `_admin_event_matches_web` + the ratatoskr `AdminEvent` domain type UNCHANGED (preserves
+  the web surface per § Out of scope). **Rejected alternative:** yield SDK `AdminEvent`s
+  through and rewire the web filter for `admin_id`/None/nan (the slice-2 turn-stream
+  precedent) — heavier web churn + scatters the None/nan hardening through the filter;
+  re-wrap localizes it. The ratatoskr `AdminEvent` dataclass stays in `sse_client.py` this
+  slice (imported by `wt` + the web); its home moves in slice-7 teardown if `sse_client.py`
+  is retired.
+- **Admin-stream error mapping (reuses the § Error map stream rows).** The SDK admin
+  stream raises `ApiError("admin_stream_failed", status=…)` on a NON-200 open (NOT
+  `ConnectFailed` — a gotcha the web integration test caught that the unit fake could not)
+  → `SseConnectFailed`; and `ConnectionDropped` on a connect-time transport failure
+  (cursor None) OR a mid-stream drop / the long-lived stream's resumable EOF (cursor set)
+  → `SseConnectionDropped`. The SDK admin stream is best-effort (skips malformed frames —
+  no `Malformed*`), as was the retired hand-rolled path; the web endpoint's existing
+  `except (…, MalformedSseId, MalformedSseData)` stays a harmless defensive superset
+  (pre-existing, not introduced here).
+
 ## Out of scope
 
 - Bifrost PROVIDER planes (memory/affect) — hand-rolled, ADR-0009, untouched.