feat(daemon): add shared cross-process connection registry

The daemon forks one child per accepted connection, so per-module and
per-source accounting must live in state shared across the children. Add a
fixed-size registry carved from an anonymous shared mapping
(mmap(MAP_SHARED|MAP_ANONYMOUS)) created before the accept loop: a slot
lifecycle (FREE/CLAIMED/REGISTERED) with parent claim/reclaim and a
lock-free, open-addressed per-source table for the per-host occupancy and
the shared auth-failure counter. C11 atomics only; no pthread locks across
fork.

Unit tests cover slot exhaustion, the module/host caps, pid reclaim and
fork-shared visibility.

src/shared/daemon_limits.h

@@ -0,0 +1,102 @@
+#ifndef DAEMON_LIMITS_H
+#define DAEMON_LIMITS_H
+
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+
+/* Cross-process daemon connection registry.
+ *
+ * The daemon listener forks ONE child per accepted connection, so any
+ * per-module / per-source accounting must live in state shared across the
+ * forked children.  This module owns a fixed-size registry carved out of an
+ * anonymous shared mapping (mmap(MAP_SHARED | MAP_ANONYMOUS)) created by the
+ * accept-loop PARENT before it forks; every child inherits the mapping (and the
+ * pointer to it) across fork().
+ *
+ * Rules:
+ *  - ONLY C11 atomics (atomic_*); never mtx_t/pthread locks, which can deadlock
+ *    in a forked child if another thread held them at fork time.
+ *  - No heap allocation after fork: the mapping is fixed-size and all access is
+ *    atomic load/store/CAS over preallocated arrays.
+ *
+ * Slot lifecycle (the parent reclaims even when a child is SIGKILLed):
+ *   FREE --(parent claim_slot)--> CLAIMED
+ *   CLAIMED --(child register)--> REGISTERED
+ *   any --(parent reclaim)--> FREE
+ * The child records its module index and per-source bucket into the slot before
+ * publishing REGISTERED; the parent's SIGCHLD handler matches the reaped pid to
+ * the slot and, when REGISTERED, decrements the module/per-source counters.
+ * A child killed before registering holds no counts, so reclaiming a CLAIMED
+ * slot only frees the slot.
+ *
+ * Per-source identity is the normalized numeric peer IP (IPv4-mapped IPv6 is
+ * already collapsed to IPv4 by utils_fd_peer_ip); it is interned into an
+ * open-addressed, linear-probing table keyed by a 64-bit hash.  The same table
+ * also carries the cross-process auth-failure counter and lockout deadline.
+ */
+
+typedef struct DaemonLimitRegistry DaemonLimitRegistry;
+
+/* Result of a per-connection admission check. */
+typedef enum {
+  DAEMON_LIMIT_OK = 0,      /* admitted; slot is now REGISTERED */
+  DAEMON_LIMIT_MODULE_FULL, /* module's `max connections` cap reached */
+  DAEMON_LIMIT_HOST_FULL,   /* global `max connections per host` cap reached */
+  DAEMON_LIMIT_UNAVAILABLE, /* registry/slot unusable (caller fails open) */
+} DaemonLimitResult;
+
+/* Bounds for registry sizing.  A slot is one concurrently live child. */
+#define DAEMON_LIMITS_MIN_SLOTS 16
+#define DAEMON_LIMITS_MAX_SLOTS 65536
+#define DAEMON_LIMITS_MAX_HOST_SLOTS 65536
+#define DAEMON_LIMITS_NO_SLOT (-1)
+
+/* Create the shared registry in the calling (parent) process.  `max_slots` is
+ * the number of concurrently live children to track (clamped to
+ * [DAEMON_LIMITS_MIN_SLOTS, DAEMON_LIMITS_MAX_SLOTS]); `module_count` is the
+ * number of daemon modules (clamped to >= 1); `per_host_cap` and the lockout
+ * pair come from the daemon config (0 disables).  Returns NULL on failure (e.g.
+ * mmap allocation); callers must degrade gracefully (global cap + ACLs still
+ * apply). */
+DaemonLimitRegistry* daemon_limits_create(int max_slots, int module_count, int per_host_cap,
+                                          int lockout_threshold, int lockout_duration_sec);
+
+/* Unmap the registry.  Only the creating process may call this. */
+void daemon_limits_destroy(DaemonLimitRegistry* registry);
+
+/* Parent side: reserve a slot for the next fork.  Returns the slot index or
+ * DAEMON_LIMITS_NO_SLOT when every slot is in use. */
+int daemon_limits_claim_slot(DaemonLimitRegistry* registry);
+/* Parent side: record the forked child's pid in a claimed slot. */
+void daemon_limits_set_slot_pid(DaemonLimitRegistry* registry, int slot, long pid);
+/* Parent side: release a slot, decrementing the module/per-source counters when
+ * the slot was actually REGISTERED.  Idempotent. */
+void daemon_limits_reclaim_slot(DaemonLimitRegistry* registry, int slot);
+/* Parent SIGCHLD side: reclaim the slot owned by `pid` (no-op when not found). */
+void daemon_limits_reclaim_pid(DaemonLimitRegistry* registry, long pid);
+
+/* Child side: admit the connection for `module_index` from `peer_ip`.  Always
+ * tracks the module/per-source occupancy (so the parent's reclaim is
+ * symmetric); when `module_cap` > 0 it additionally enforces the per-module
+ * cap.  Returns DAEMON_LIMIT_OK and publishes the slot, or a refusal reason. */
+DaemonLimitResult daemon_limits_register(DaemonLimitRegistry* registry, int slot, int module_index,
+                                         const char* peer_ip, int module_cap);
+
+/* Child side: true when `peer_ip` is currently locked out after too many failed
+ * authentications.  `seconds_remaining` may be NULL. */
+bool daemon_limits_auth_locked(DaemonLimitRegistry* registry, const char* peer_ip,
+                               int* seconds_remaining);
+/* Child side: count one failed authentication for `peer_ip`; once the threshold
+ * is reached the source is locked out for the configured duration. */
+void daemon_limits_auth_record_failure(DaemonLimitRegistry* registry, const char* peer_ip);
+/* Child side: clear the failure counter/lockout for a source that authenticated
+ * successfully (no-op when the source has no table entry). */
+void daemon_limits_auth_record_success(DaemonLimitRegistry* registry, const char* peer_ip);
+
+/* Pure helper: 64-bit FNV-1a hash of a numeric peer IP plus its family, used to
+ * index the per-source table.  *ok is set false (and 0 returned) for a NULL or
+ * non-numeric address.  Exposed for unit testing. */
+uint64_t daemon_limits_host_hash(const char* peer_ip, bool* ok);
+
+#endif