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Adds an opt-in, in-memory aggregator of recent connection-rejection events (TSMP rejects received from peers, outbound TSMP rejects we emit on ACL-blocked inbound flows, and pendopen timeouts) keyed by (direction, proto, peer-address, reason). The aggregated data is exposed over a new debug-rejects LocalAPI endpoint and a GET /debug/rejects c2n endpoint, intended for future GUI/CLI consumption when diagnosing why a connection failed. Architecture: - net/connreject holds the data types and a per-LocalBackend Aggregator (LRU-bounded, default 256 entries on desktop / 32 on mobile, per direction). - feature/connreject is a self-registering ipnext.Extension that owns one Aggregator per LocalBackend, installs note callbacks on the tundev and engine, subscribes to OnSelfChange to flip the runtime gate, and serves the LocalAPI/c2n endpoints. - wgengine.Engine and *tstun.Wrapper each gain a SetConnRejectNote setter; data-plane sites use a single atomic.Pointer load + nil check, so the cost when no consumer is installed is one MOV. Gating: - Compile-time: ts_omit_connreject build tag (standard feature/buildfeatures + condregister plumbing). Trims ~41 KB. - Runtime: tailcfg.NodeAttrConnReject node attribute, off by default at the control plane. May be removed once the feature is enabled by default. Updates CapabilityVersion to 139 (clients understand NodeAttrConnReject and can serve GET /debug/rejects). Adds Proto/Src/Dst accessors on flowtrack.Tuple (used by pendopen to construct events without exposing the tuple's internals to the aggregator). Updates #1094 Updates #14802 Signed-off-by: James Tucker <james@tailscale.com>