fix: treat TLS errors as failover-worthy and remove unsafe start-block fallback

[vietddude]

Problem

Production logs for bsc_mainnet-1 revealed two distinct bugs:

WRN Unknown provider error type ... error="...remote error: tls: internal error"
DBG Provider failed but not switching ... state=healthy consecutive_errors=0
...
WRN Cannot get latest block from chain or KV, using config.StartBlock startBlock=74588255

1. TLS errors didn't trigger failover. remote error: tls: internal error matched no pattern in analyzeError() → classified as generic_error (markUnhealthy=false) → the provider stayed healthy and the indexer kept hammering the broken endpoint.
2. Stale-block rewind risk. When both the chain RPC and KV failed, determineStartingBlock() fell straight back to config.StartBlock. Root cause: GetLatestBlock collapsed "key absent (first run)" and "store down" into the same error, so a transient outage at boot could silently rewind the indexer to an old block and re-index everything.

Changes

1. TLS errors are now failover-worthy (internal/rpc/failover.go)

• Added a tls_error pattern (tls: internal error, tls handshake, handshake failure, remote error: tls) with markUnhealthy: true, 2m cooldown → the provider is cooled down and rotated away.

2. Distinguish "not found" from "store error" (pkg/store/blockstore/store.go)

• GetLatestBlock returns (0, nil) for a missing key (kvstore.ErrKeyNotFound — Consul/Badger), and only propagates genuine store errors.

3. Drop the config.StartBlock fallback, anchor on chain head (internal/worker/regular.go)

• Rewrote determineStartingBlock: if KV has a prior block → resume (+ queue the gap for catchup); cold start / store down → waitForChainHead() waits until the chain responds (honoring ctx).
• getLatestBlockWithRetry does bounded retries on the resume-from-KV path.
• No more rewinding to config.StartBlock (config field kept, now unused).

4. Cleanup / refactor (internal/worker/regular.go)

• Extracted queueCatchupRanges, shared by determineStartingBlock and skipAheadIfLagging (removed duplication).
• Split processRegularBlocks (~90 → ~45 lines) into processBatch + commitProgress.
• Removed dead code checkContinuity (only referenced by a test).

Tests

• internal/rpc/failover_test.go: TLS error blacklists the provider (tls_error).
• internal/worker/regular_test.go: determineStartingBlock cases — store down + chain up, wait-for-head (chain fails transiently then recovers), cold start, ctx-cancel → 0, resume from KV.
• pkg/store/blockstore/store_test.go: GetLatestBlock returns (0, nil) on missing key, propagates real store errors.

go build ./..., go vet, and all tests in the affected packages pass.