feat(cluster): wire IngestHandler on readers for query freshness

[xe-nvdk]

Summary

• Reader nodes now apply replicated WAL entries to their local ArrowBuffer, enabling near-real-time query freshness across the cluster
• buildReplicationIngestHandler() parses WAL envelope, decodes both columnar and row msgpack formats, writes via WriteColumnarDirectNoWAL (avoids WAL double-write since receiver's LocalWAL already handles persistence)
• Added IngestHandlerFunc adapter type (follows http.HandlerFunc pattern)
• Extracted wal.ParseEnvelope() as shared utility — replaces inline envelope parsing in both wal/reader.go and coordinator.go
• Foundation for feat: query in-flight buffer data (zero-latency reads) #249 (query in-flight buffer data)

Data flow (after this change)

Writer: Ingest → ArrowBuffer → WAL → replication hook → Sender → TCP → Reader
Reader: Receiver → LocalWAL (persistence) + IngestHandler → ArrowBuffer (query freshness)

Test plan

• go build ./cmd/... ./internal/... passes
• go test ./internal/cluster/... passes
• go test ./internal/wal/... passes
• Post-implementation review: nil safety, envelope parsing, format detection

[gemini-code-assist]

Code Review

This pull request implements reader query freshness by enabling reader nodes to apply replicated WAL entries directly to their local in-memory ArrowBuffer. It introduces an IngestHandler within the cluster coordinator to decode both columnar and row-formatted WAL payloads and write them to the buffer. The review feedback highlights several improvement opportunities: ensuring the row-format decoder correctly identifies measurement metadata keys, protecting access to the ingest buffer with read locks to prevent data races, and optimizing the rowsToColumns conversion to filter out metadata and improve efficiency.