docs(capture): make Q4 audit-sink-down posture classification-aware (#49)

Reconsidered Q4: instead of one global degrade-and-warn, the posture now
inherits from effective classification (Q1):
- confidential + audit-sink-down -> hard-refuse (no buffer; removes the
  buffer-integrity question for confidential data)
- internal/public + audit-sink-down -> degrade-and-warn + durable local
  buffer + ntfy + reconcile-on-recovery
- floor (all tiers): refuse if nothing can record the audit
Updated the I5 Gherkin scenarios + obligations row to match. Couples Q4
to the Q1 classification spine -> one coherent sensitivity model.

specs/capture-bdd-spec.md

@@ -47,7 +47,7 @@ beyond the I5 audit log.
 | **I2 deliberate acceptance** | The *distributed-library* form opens no new acceptance. IF a central relay node is deployed, its cross-harness reach MUST be entered in `infra/docs/security-baseline.md` with Why-accepted / Revisit-if before it ships. |
 | **I3 GitOps reconcilability** | IF `capture` runs as a deployed service, its manifest lives under `infra/k3s/apps/**` (sovereign source, Flux-reconciled). No untracked runtime. |
 | **I4 decisions captured** | The distributed-vs-central decision and the intent-named-verb pattern are recorded (ADR + brain). |
-| **I5 auditability** | Every capture emits a request-level audit record to the alloy/loki substrate: actor/principal, harness, items written, timestamp. |
+| **I5 auditability** | Every capture emits a request-level audit record (actor/principal, harness, items written, timestamp) to the alloy/loki substrate. **Classification-aware degradation** (§4.4): confidential + sink-down → hard-refuse; internal/public + sink-down → durable local buffer + ntfy + reconcile. Floor: refuse if nothing can record the audit. |
 
 ---
 
@@ -147,24 +147,33 @@ Feature: Capture session value uniformly across harnesses
     Then the would-be receipt is returned
     And nothing is written anywhere
 
-  # --- I5: auditability is non-optional, but degrades rather than blocks ---
-  Scenario: Capture proceeds under a durable local audit buffer when the central sink is down
-    Given the central audit substrate (loki) cannot be written to
+  # --- I5: auditability is classification-aware (confidential fails closed) ---
+  Scenario: Confidential capture hard-refuses when the central audit sink is down
+    Given the effective classification is "confidential"
+    And the central audit substrate (loki) cannot be written to
+    When capture is invoked
+    Then the capture is refused before any write
+    And the reason names the auditability invariant
+    # Confidential work must be centrally auditable at write time — no buffered exception.
+
+  Scenario: Internal capture degrades to a durable local buffer when the sink is down
+    Given the effective classification is "internal" or "public"
+    And the central audit substrate (loki) cannot be written to
     When capture is invoked
     Then the capture proceeds
     And the audit record is written to a durable LOCAL fallback buffer
     And an ntfy alert is emitted naming the degraded audit state
     And the receipt flags that audit was buffered locally, not centrally recorded
-    # Never UN-audited: the write is buffered durably and reconciled on recovery.
 
   Scenario: Locally buffered audit records reconcile to the central sink on recovery
-    Given audit records were buffered locally during a sink outage
+    Given internal-tier audit records were buffered locally during a sink outage
     When the central audit substrate becomes reachable again
     Then the buffered records are replayed to the central sink
     And the local buffer is cleared only after confirmed central write
 
-  Scenario: Capture refuses if even the local audit buffer cannot be written
-    Given neither the central sink nor the local fallback buffer can be written
+  Scenario: Even internal capture refuses if neither sink nor local buffer can be written
+    Given the effective classification is "internal" or "public"
+    And neither the central sink nor the local fallback buffer can be written
     When capture is invoked
     Then the capture is refused
     And the reason names the auditability invariant
@@ -211,13 +220,23 @@ binding design decisions for the build.
    reach **must be entered in `infra/docs/security-baseline.md`** with Why-accepted / Revisit-if
    **before it ships** (I2). That ledger entry is v1 work, not a follow-up.
 
-4. **Audit-sink-down — degrade-and-warn, with a durable local buffer + reconcile-on-recovery.**
-   If the central audit sink (loki) is unreachable, capture **proceeds** rather than blocking
-   (availability), BUT the audit record is written to a **durable local fallback buffer** and an
-   **ntfy alert** fires. Buffered records **reconcile to the central sink on recovery**; the local
-   buffer clears only after confirmed central write. **Floor:** if *neither* the central sink nor the
-   local buffer can be written, capture **refuses** — degrade-and-warn never means *un*-audited.
-   - Rationale: keeps capture available during an observability outage while preserving I5 — the
-     write is always recorded *somewhere durable*, just not centrally until reconciliation. This is
-     the difference between an I5 violation and an I5-compliant degraded mode, and it must be
-     presentable as such to a due-diligence client.
+4. **Audit-sink-down — classification-aware: confidential fails closed, internal/public degrades.**
+   The posture inherits from the effective classification (decision 1), so there is one coherent
+   sensitivity model rather than a separate availability policy:
+   - **Confidential + central audit sink unreachable → hard-refuse.** No buffer, no proceed.
+     Confidential work must be centrally auditable *at write time*; "buffer and reconcile later"
+     introduces a buffer-integrity question (can a write tamper with its own pending audit record?)
+     that must not exist for confidential data. The simplicity of "refuse" is itself the assurance
+     asset — trivially true, nothing to poke holes in.
+   - **Internal / public + central sink unreachable → degrade-and-warn** with a durable local buffer
+     + ntfy alert + reconcile-on-recovery (the earlier Q4 design, now scoped to lower tiers). Keeps
+     capture available for your own homelab work during an observability outage; negligible risk
+     since the buffered record is still durable and the data isn't client-confidential.
+   - **Floor (all tiers):** if *nothing* — neither central sink nor (for internal/public) the local
+     buffer — can record the audit, capture **refuses**. No tier writes wholly un-audited.
+   - Rationale: matches assurance cost to data sensitivity, exactly as the I1/sovereignty model
+     does for placement. Presentable to a due-diligence client as "audit posture is
+     classification-aware: confidential fails closed, internal degrades gracefully" — which
+     demonstrates the judgment, not just a binary. Couples Q4 to Q1's classification machinery
+     (being built anyway) and removes the buffer-integrity rabbit hole for the only tier where it
+     mattered.