Add deployment-managed cache writer

docs/adr/0017-defer-cache-access-policy-to-deployment.md

@@ -0,0 +1,59 @@
+# ADR-0017: Defer cache access policy to the deployment
+
+**Status:** Accepted
+**Date:** 2026-09-11
+
+## Context
+
+Canonical chunk plans and invocation checkpoints are reconstructible cache
+state. They may include source-derived content, so access and retention matter,
+but Notarius cannot determine whether a deployment needs a private cache or a
+cache shared by a trusted group. The current application-enforced `0700` and
+`0600` modes prevent ordinary Unix provisioning from making that choice.
+
+[ADR-0005](0005-cache-canonical-chunk-plans-by-source.md) governs canonical
+chunk-plan reuse, while [ADR-0006](0006-separate-output-cache-and-debug-state.md)
+established the output, cache, and debug separation refined by
+[ADR-0007](0007-separate-checkpoint-recording-from-reuse.md). None of those
+decisions requires Notarius to own exact cache access modes.
+
+## Decision
+
+Deployments own cache access policy through provisioned directories, process
+umask, setgid inheritance, and default ACLs. Notarius requests conventional
+maximum creation modes for cache entries and leaves the kernel to select their
+effective permissions.
+
+Notarius continues to own confined paths, symlink rejection, validation,
+atomic publication, cleanup, and recovery. Those correctness boundaries are
+independent from deciding which accounts may access cache state.
+
+Source-derived cache contents can be sensitive. Operators own their access and
+retention policy, while API credentials remain outside cache payloads under the
+existing secret boundary. Existing cache inode modes are not migrated
+automatically.
+
+This decision does not supersede ADR-0005, ADR-0006, or ADR-0007.
+
+## Alternatives considered
+
+- Continue enforcing private `0700` directory and `0600` file modes. This
+  makes one deployment choice for every operator and prevents trusted-group
+  provisioning.
+- Add configurable Notarius permission modes or a private/group cache setting.
+  This preserves application ownership of a deployment concern and expands the
+  public configuration surface without a durable policy reason.
+- Recursively converge existing cache ownership, modes, or ACLs. This would
+  make upgrades mutate operator-managed state and cannot safely infer an
+  intended policy.
+
+## Consequences
+
+Operators can use a restrictive umask for private cache state or provision a
+setgid/default-ACL cache root for trusted-group access. They must deliberately
+recreate or administratively migrate pre-existing cache trees when their
+existing modes do not match the desired deployment policy.
+
+Notarius does not add user or group management, ACL mutation, permission
+configuration, or recursive cache migration. Output and debug permission
+contracts remain separate from this decision.

docs/roadmap/implementation.md

@@ -38,6 +38,8 @@ Every stage must preserve these decisions:
 
 ## Stage 1: Record The Decision And Add An Umask-Respecting Atomic Writer
 
+**Completed:** 2026-09-11
+
 ### Goal
 
 Record the durable ownership boundary and add the narrow filesystem primitive