Epic: per-tenant namespace isolation and fast teardown (SDK-native)

[somfornot]

Design: #554 (docs/explanation/tenant-namespace-isolation.md).

Per-tenant data namespace isolation + fast tenant-offline teardown on the SDK-native direct data plane. Tenant = the SDK-declared TenantId, treated as a trusted partition key; each tenant has a private namespace backed by its own origin bucket + credentials (coordinator-distributed registry). The object-store gateway is out of scope.

Decisions (locked)

1. Boundary = SDK-declared TenantID, trusted partition — not bucket/prefix, not a gateway principal; not a hard security boundary (native-plane auth is an additive follow-on).
2. Private per-tenant namespace — same path → different data per tenant; tenant participates in origin addressing.
3. Per-tenant origin — each tenant has its own bucket + credentials via a coordinator-distributed registry.

Delivery slices

• talon-core: TenantOrigin record, tenant-scoped identity, registry trait #556 — talon-core: TenantOrigin record, tenant-scoped identity, registry trait
• coordinator: tenant origin registry store, distribution, management API #557 — coordinator: registry store, distribution, management API
• worker: per-tenant cache shard + tenant-scoped read/serve path #558 — worker: per-tenant cache shard + tenant-scoped read/serve path
• worker: miss-path origin resolution + per-tenant credentials (fail-closed) #559 — worker: miss-path origin resolution + per-tenant credentials (fail-closed)
• worker: DropTenant fast teardown (rename + background unlink + fence) #560 — worker: DropTenant fast teardown (rename + background unlink + fence)
• wire/SDK: tenant-scoped PUT/DELETE + tenant-scoped FUSE mount #561 — wire/SDK: tenant-scoped PUT/DELETE + tenant-scoped FUSE mount
• placement: fold tenant into the Maglev hash (C/Java/Python parity) #562 — placement: fold tenant into the Maglev hash (C/Java/Python parity)
• observability: per-tenant residency and hit/miss #563 — observability: per-tenant residency and hit/miss

Note: the design (#554) is under review; slices may adjust.

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