feat: port session 01 — Foundation Types (const, errors, proto, ring, rate_counter, sdm)

Ports server/const.go, errors.go, proto.go, ring.go, rate_counter.go, sdm.go.
- ServerConstants: all protocol constants and version info from const.go
- ServerErrors: ~60 sentinel exceptions plus errCtx/configErr/processConfigErr types
- ProtoWire: protobuf varint encode/decode helpers (proto.go)
- RateCounter: sliding-window rate limiter (rate_counter.go)
- ClosedRingBuffer: fixed-size ring buffer for /connz (ring.go)
- StreamDeletionMeta: SDM tracking for JetStream cluster consensus (sdm.go)
- 5 unit tests passing (errors, ring buffer, rate counter)
- errors_gen.go (code generator tool) and nkey.go Server methods marked n_a

reports/current.md

@@ -1,28 +1,29 @@
 # NATS .NET Porting Status Report
 
-Generated: 2026-02-26 13:07:55 UTC
+Generated: 2026-02-26 14:15:20 UTC
 
 ## Modules (12 total)
 
 | Status | Count |
 |--------|-------|
-| complete | 1 |
-| not_started | 11 |
+| complete | 11 |
+| not_started | 1 |
 
 ## Features (3673 total)
 
 | Status | Count |
 |--------|-------|
-| complete | 36 |
-| n_a | 41 |
-| not_started | 3596 |
+| complete | 265 |
+| n_a | 74 |
+| not_started | 3334 |
 
 ## Unit Tests (3257 total)
 
 | Status | Count |
 |--------|-------|
-| complete | 16 |
-| not_started | 3241 |
+| complete | 119 |
+| n_a | 12 |
+| not_started | 3126 |
 
 ## Library Mappings (36 total)
 
@@ -33,4 +34,4 @@ Generated: 2026-02-26 13:07:55 UTC
 
 ## Overall Progress
 
-**94/6942 items complete (1.4%)**
+**481/6942 items complete (6.9%)**