fix(metrics): remove deprecated FECShortShards and add new KCP metrics

[moralpriest]

What

Remove deprecated KCP_FECShortShards metric and add 7 new KCP SNMP metrics, vendoring kcp-go v5.6.72 to support them.

Why

• kcp-go v5.6.72 removed the FECShortShards field from its Snmp struct, causing a compile error against the current vendored copy (~2020)
• The new version adds 7 new fields: FECFullShardSet, FECShardSet, FECShardMin, RingBufferSndQueue, RingBufferRcvQueue, RingBufferSndBuffer, OOBPackets
• This PR vendors kcp-go v5.6.72 to make all 30 Snmp fields available, preparing for the eventual go.mod modernization

Changes

Area	Change
metrics/metrics.go	Removed KCP_FECShortShards; added 7 new metrics; grouped imports
vendor/github.com/xtaci/kcp-go/v5/	Replaced old vendored copy with v5.6.72 (29 .go files)
vendor/github.com/templexxx/	Deleted dead deps (only old kcp-go imported them)
go.mod / go.sum	Not modified — still go 1.17, no go.sum

Verification

• API compatibility verified: all kcp-go symbols used by metrics.go and p2p/controller.go have identical signatures in v5.6.72
• go build ./metrics/ ./p2p/ — passes
• go vet ./metrics/ ./p2p/ — passes
• go test ./metrics/ ./p2p/ — passes
• Runtime render test confirms all 9 KCP metrics appear and KCP_FECShortShards is absent
• go 1.17 directive does not block kcp-go v5.6.72's generics (each module uses its own go directive)

Notes

• Preparatory work for the jrpc2 migration PR (chore(deps): migrate jrpc2 v0 → v1.3.5 #19) — once this merges, commit 018a95f6 can be dropped from chore(deps): migrate jrpc2 v0 → v1.3.5 #19 and go mod vendor re-run to sync the vendor directory
• kcp-go v5.6.72 uses Go generics (NewRingBuffer[T]), which requires Go 1.18+ — but this is per-module: kcp-go's own go.mod says go 1.24.0, so it compiles fine with go 1.17 in the main module