feat: context-scoped multi-workspace routing (workspace() scope + routing exporter)

[PabloPardoGarcia]

RIUS-646. One process, many workspaces: init(workspaces={alias: api_key}) plus with rius.workspace(alias): routes every span started in scope (observe, generations, sessions, and auto-instrumented spans, which inherit the OTel context) to that workspace's destination. Spans outside any scope keep using the default key.

Design mirrors sessions end to end:

• workspace(alias) context manager on an OTel context key (copy of session())
• WorkspaceSpanProcessor stamps a transient rius.workspace attribute at span start (copy of SessionSpanProcessor), and warns when a span starts under a different alias than its parent's stamp: one trace, one workspace (the backend derives the workspace from the key)
• RoutingSpanExporter sits innermost in the export chain (masking and export-health wrap the fan-out), partitions each batch by the attribute, strips it via the same copy-on-write discipline as MaskingSpanExporter (it never reaches the wire), and forwards each partition to a lazily-built per-key OTLP exporter; flush delivery and shutdown fan out

Decisions from the ticket, implemented:

• Unknown alias routes to the DEFAULT destination with a once-per-alias warning: data is kept in the vendor's own workspace and can never leak to another customer
• rius.workspace joined PENDING_IDENTITY_ATTRIBUTES, so a partial-span snapshot routes to the same workspace its final span will (covered by test)
• Dynamic registration: client.register_workspace(alias, key) / module-level rius.register_workspace for keys fetched at runtime; workspaces={} opts into routing with no static routes; re-registering an alias rotates its key
• Heartbeat unchanged: liveness reports under the default workspace (v1 decision)
• workspace_exporter_factory injection point for tests, matching the repo's DI style

12 new tests in tests/test_workspace.py; full suite 240 passed, ruff + mypy strict clean.

[PabloPardoGarcia]

Verified end-to-end against staging: init() with a pablo-test key as default and a B-test key under alias "acme"; a bare span landed only in pablo-test, the workspace("acme")-scoped root+child trace landed only in B-test, and ClickHouse shows the stored spans carry only openinference.span.kind — the rius.workspace attribute never reached the wire. flush() reported delivery success for the fan-out.