Lower bound fixes + gold lower bounds

[KirillPamPam]

Gold lower bounds + lower-bound detection fixes

Summary

This PR fixes several correctness bugs in EVM lower-bound detection and adds a fast
"gold lower bound" short-circuit that lets us skip the binary search entirely when a
chain ships a known-oldest data point. It also splits the monolithic EVM detector into
per-type files and hardens the retry behaviour of the shared search.

Gold lower bounds

chains.yaml now carries an optional lower-bounds section per chain, parsed into new
ChainLowerBounds / GoldLowerBound types (pkg/chains/chains.go) and threaded through
ConfiguredChain. A gold bound is a known-oldest {block, hash} pair for a chain. This
data already ships in the bundled chains.yaml submodule — it was simply never parsed, so
the change is purely additive (bitcoin and other chains without the section stay nil).

When a chain has a gold hash for tx or receipts, the detector issues a single
eth_getTransactionByHash / eth_getTransactionReceipt probe for that ancient hash:

• Hit → the upstream serves that oldest datum, so it is fully archival and the bound
  resolves to 1 with no binary search.
• Miss (null result, no-data error, or any other error) → falls back to the normal
  search path.

This is implemented in the new gold_bound.go and short-circuits at the top of
DetectLowerBound. EVM detectors now receive the full *chains.ConfiguredChain (instead
of just chains.Chain) so they can read the configured bounds; LowerBoundResults was
exported so the short-circuit reuses the same per-supported-type fan-out as the search.

Lower-bound search fixes

The shared LowerBoundSearchCalculator (lower_bound_search.go) was rewritten to fix
real detection bugs:

• Genesis-hole bug. The old search trusted block 1 whenever it was retained. If a node
  retained genesis but had a hole above it, it reported a bogus bound of 1. The search now
  converges on the real lower boundary of the contiguous retained range regardless of
  whether genesis happens to be present.
• Offset search for tx/receipts. Historical blocks may legitimately contain no
  transactions, which made a naive "first tx of the middle block" probe give up too early.
  A new offset variant (detectWithOffset / shiftLeftAndSearch) scans downward up to
  maxOffset (20) blocks from a no-data midpoint to tolerate sporadic empty/missing blocks;
  a hole wider than the offset is abandoned and the search moves higher. Tx and receipts
  detectors use this variant; block/state/proof use the plain binary search.
• No-data errors treated as "no data here". Archival/pruning gaps surface as a wide
  range of upstream error messages. These are now classified (no_data_errors.go) and
  treated as "no data at this height, search higher" rather than aborting the whole
  detection. Only genuinely unexpected failures propagate.
• Convergence guard. On a non-trivial chain (latest > 10) where nothing is ever
  confirmed, detection now fails cleanly so the processor keeps the previously cached
  bound instead of publishing a bogus 1. Tiny chains still resolve to 1.
• Cached-bound reuse. A previously found bound is re-confirmed with a single probe
  before any re-search, keeping steady-state cost to one call per cycle.
• Retry policy. Probes and latest-height fetches now retry with exponential backoff
  (up to 30 attempts, 1s → 1m) instead of the previous fixed 3× / 100ms, so transient
  upstream errors no longer poison a detection cycle. SetSearchRetryPolicy lets tests
  run with negligible backoff.

Refactor: split evm_bounds by bound type

evm_lower_bound.go was ~470 lines mixing construction, five per-type probes, JSON
helpers, and the error-hint list. It is now split into focused files so each bound type
reads as a self-contained unit, with the shared wiring left behind:

• block_bound.go, state_bound.go, tx_bound.go, receipts_bound.go, proof_bound.go
  — one constructor + probe per data type.
• evm_util.go — block/tx envelope parsing and hex/JSON helpers.
• no_data_errors.go — the no-data error classification and hint list.
• gold_bound.go — the gold short-circuit.
• evm_lower_bound.go — construction, probe dispatch, fetchLatestHeight, and the
  JSON-RPC call helper.