polygon: Use BorSpansIndex to calculate span id at block number

[antonis19]

This PR changes the way the span id at a given block number is calculated. Whereas before, the span id was calculated via the SpanIdAt(blockNum) function, which did a simple arithmetic calculation to get the span id, with the changes in this PR a spanRangeIndex RangeIndexer is used to calculate the span at given block via a span.StartBlock -> (span.Id, span.EndBlock) MDBX table (kv.BorSpansIndex for Spans, and kv.BorProducerSelectionsIndex for BorProducerSelections ).

The span id for a given blockNum is now calculated as:
max span.Id such that blockNum >= span.StartBlock && blockNum <= span.EndBlock

The reasons for this change are 2:

1. In the upcoming VeBlop hardfork (https://github.com/maticnetwork/Polygon-Improvement-Proposals/blob/main/PIPs/PIP-64.md) spans could overlap if there is a producer rotation before the current span completes. E.g. Span 0 is [0 ; 6399], but in block 5 there is a new span with a different producer (due to the block producer of span 0 going down), so Span 1 is [5; 12,799] . Therefore, the span at block 10 for example is no longer Span 0, but Span 1 instead. For this reason, a more flexible approach is needed to determine the span id for a given block.

2. In the old code a hardcoded span length of 6400 blocks was used. This worked fine for Bor mainnet and Amoy, but for Kurtosis testing, Polygon uses a configurable span length (default now is 128 blocks). This means that the Erigon node in the kurtosis setup was misbehaving. With this PR, the kurtosis Polygon tests can work no matter what the span length is, since the span id calculation is more flexible.

The old SpanIdAt() function is deprecated and only kept in some obsolete tests that use HeimdallSimulator, which can be removed in the future.

[Copilot]

Pull Request Overview

This PR introduces a fundamental change to how span IDs are calculated for block numbers in Polygon. Instead of using a simple arithmetic function, it implements a flexible BorSpansIndex MDBX table to handle overlapping spans that can occur during producer rotations in the upcoming VeBlop hardfork.

Key changes:

• Replaces hardcoded span length calculations with a dynamic range indexer system
• Implements support for overlapping spans where a new span can start before the previous one ends
• Adds comprehensive testing for the new span range index functionality

Reviewed Changes

Copilot reviewed 35 out of 36 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
erigon-lib/event/observers.go	Updates Observer interface to accept context parameter
polygon/heimdall/span_range_index.go	Introduces new span range indexer for flexible span ID lookups
polygon/heimdall/types.go	Updates snapshot extraction to use dynamic span indexing
polygon/heimdall/service.go	Modifies service to use database-backed span lookups
db/kv/tables.go	Adds new MDBX tables for span and producer selection indexing
eth/backend.go	Updates observer notifications to include context

Comments suppressed due to low confidence (1)

polygon/heimdall/span_range_index.go:113

• The logic is inverted - when Put returns an error, the function returns (false, err) but the comment says "we can continue updating". When Put succeeds (err == nil), it returns (true, nil) but says "we need to stop". The return values should be swapped to match the intended behavior.

	if valuePair == nil {

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[mh0lt]

Why are you exposing the heimdallMdbxStore here - it feels like it should be accesses via the heimdall store by the components that need it rather than added into the back-end code. We're trying to remove these dependencies not all more.

[taratorio]

waiting on #16683 (comment)