transport+visor: retire on-disk CSV transport-log store; serve history from stats bbolt

[0pcom]

Summary

Retire the per-transport CSV log store. On a public visor it was the dominant alloc-rate symbol after #2420 landed — pprof showed transport.fileTransportLogStore.writeToCSV at ~78% of cumulative bytes allocated, going through gocsv reflection-based marshalling for every packet on every ManagedTransport. The CSV files were also strictly narrower than data we already publish: the pkg/visor/stats bbolt store carries the same bandwidth + latency snapshots, with daily rollups already feeding the CXO publisher subscribed by the TPD aggregator.

This PR removes the file-backed store entirely and re-points GetTransportLogs() (the only consumer of the CSVs — used by skywire-cli tp -b N for bandwidth history) at the bbolt stats store.

Changes

Removed code

• pkg/transport/log.go — drop fileTransportLogStore (writeToCSV / readFromCSV / recoverCSV / repairCSVFile / cleanLogs). Drop the parallel fileLatencyLogStore along with its interface LatencyLogStore, inMemoryLatencyLogStore, LatencyCsvEntry — the manager held a LatencyLogStore field but nothing in the codebase ever called Record on it, pure dead weight. CsvEntry and the gocsv/encoding/csv/bufio imports go with them.
• pkg/transport/manager.go — drop the unused LatencyLogStore field on ManagerConfig.
• pkg/transport/log_test.go — drop TestFileTransportLogStore.
• pkg/visor/init_transport.go — collapse the file/memory branch to always-memory; drop the latency-store init. LogStore.Type=\"file\" in existing visor configs is accepted but logged as deprecated.
• pkg/visor/logserver/api.go — drop the /transport_logs/:file HTTP route and its landing-page listing. The same data is at /stats/transports/history (live snapshot at /stats/transports). Unused tpLogPath parameter on logserver.New stays in the signature for call-site compat (init_dmsg.go passes it).
• pkg/skyenv/skyenv.go — drop the unused LatencyLogStore directory constant. TpLogStore stays since cmd/skywire-cli/commands/config/gen.go still emits it as the default LogStore.Location for backward-compatible configs.

Re-pointed read path

• pkg/visor/api_transport.go — GetTransportLogs(days int) now walks stats.Store.AllTransportRecords() and emits one entry per (transport, day) within the requested window. The CSV store's RecvBytes/SentBytes was the within-day delta (each daily file was zero-anchored at day rollover via the explicit Reset() in writeToCSV), which matches DailyRollup's delta semantics exactly. skywire-cli tp -b N aggregation is unchanged. Drop the local csvEntry parser and readTransportLogFile.

Diff

 pkg/skyenv/skyenv.go        |   9 +-
 pkg/transport/log.go        | 537 +-------------------------------------
 pkg/transport/log_test.go   |  24 --
 pkg/transport/manager.go    |   1 -
 pkg/visor/api_transport.go  |  96 +++----
 pkg/visor/init_transport.go |  37 +--
 pkg/visor/logserver/api.go  |  28 +--
 7 files changed, 68 insertions(+), 664 deletions(-)

Behavioural notes

• Live cumulative byte counters (skywire-cli tp ls, hypervisor UI per-transport bytes): same in-memory semantics as today's memory-mode users — "bytes since transport became live in this visor session." File-mode users used to keep counters across visor restarts via the daily CSV; that no longer happens. Daily totals via tp -b N are unaffected because bbolt is the source of truth.
• /transport_logs/<date>.csv HTTP endpoint is removed. Functionally replaced by /stats/transports/history?since=&until=&id= already wired into the same logserver.

Test plan

• go test ./pkg/transport/... (transport, addrresolver, handshake, tpdclient — all green)
• go test ./pkg/visor/... (visor, dmsgtracker, logserver, stats, usermanager, visorconfig — all green)
• go build ./... — whole-repo build clean
• skywire -b runs on the resulting binary
• Verify on a live public visor: rerun pprof and confirm writeToCSV is gone from the alloc profile; verify skywire-cli tp -b 7 still returns sensible per-day bandwidth aggregated from the bbolt store