Add Windows NRPT recovery circuit breaker

Windows DNS intercept mode runs an NRPT health monitor that restores the
catch-all rule and re-signals DNS Client whenever Windows stops routing
queries to the local listener. When another agent (MDM, VPN, GPO) keeps
putting NRPT back into a broken state, that loop never converges: ctrld
repeatedly calls RefreshPolicyEx, Dnscache paramchange, and flushes the
DNS cache, producing continuous flash writes and SIEM noise while never
fixing anything.

Add a recovery limiter that trips after a configurable number of
consecutive recovery flows and enters a cooldown, during which recovery
is suppressed (logged at most once every 5 minutes). Clearing the
circuit requires two consecutive stable health successes rather than
one, because a probe can pass briefly right after delete/re-add even
when the underlying NRPT state is still broken.

New [service] options gate the behavior and default to the previous
unlimited behavior:
  - nrpt_recovery_max_attempts (default 0 = unlimited)
  - nrpt_recovery_cooldown     (default 30m)

Also collapse the repeated refresh + paramchange + flush sequence into a
single signalNRPTChange() helper, and make cleanGPPath /
cleanEmptyNRPTParent only mutate the registry and report whether cleanup
happened, so callers send exactly one DNS Client change signal instead
of several. When the GP DnsPolicyConfig parent exists but is empty,
nrptProbeAndHeal now cleans it and signals once before spending the
normal policy-refresh retry budget, since those retries cannot succeed
while DNS Client is stuck in GP mode.

Add unit tests for the limiter's cooldown, stable-reset, and unlimited
paths, and document the new options and the empty-GP repro.

docs/config.md

@@ -295,6 +295,22 @@ If a remote upstream fails to resolve a query or is unreachable, `ctrld` will fo
 - Required: no
 - Default: true on Windows, MacOS and non-router Linux.
 
+### nrpt_recovery_max_attempts
+Windows DNS intercept mode uses NRPT health probes and recovery when Windows stops routing queries to the local `ctrld` listener. This limits how many consecutive recovery flows can run before `ctrld` enters a cooldown and stops making policy/Dnscache changes.
+
+Set to `0` to disable this circuit breaker and keep retrying indefinitely.
+
+- Type: integer
+- Required: no
+- Default: 0 (unlimited, current behavior)
+
+### nrpt_recovery_cooldown
+Cooldown duration after `nrpt_recovery_max_attempts` consecutive Windows NRPT recovery flows. During cooldown, `ctrld` logs the suppressed recovery and avoids additional `RefreshPolicyEx`, Dnscache `paramchange`, and DNS cache flush calls.
+
+- Type: time duration string
+- Required: no
+- Default: 30m
+
 ## Upstream
 The `[upstream]` section specifies the DNS upstream servers that `ctrld` will forward DNS requests to.
 

docs/wfp-dns-intercept.md

@@ -91,6 +91,20 @@ ctrld uses an adaptive strategy (matching [Tailscale's approach](https://github.
    the empty GP parent key. This ensures DNS Client stays in "local mode" where
    the local-path rule activates immediately via `paramchange`.
 
+### Reproducing the Empty GP Parent Case
+
+This is a production code reference, so the temporary repro script is not kept in
+the repository. For MR !942 review, the test script and exact before/after steps
+are posted in the MR discussion. The scenario to compare is:
+
+1. Run the same approved PowerShell repro script against a pre-fix build and this
+   branch with the same ctrld config.
+2. Create an empty GP NRPT parent key while ctrld is running in DNS intercept mode.
+3. Confirm pre-fix logs can spend policy refresh/paramchange retries while the GP
+   parent remains empty.
+4. Confirm post-fix logs clean the empty GP parent, send one NRPT-change signal,
+   and re-probe before normal retries.
+
 ### VPN Coexistence
 
 NRPT uses most-specific-match. VPN NRPT rules for specific domains (e.g.,