refactor(tsdb/index): drop FormatV1 reader support

[simonswine]

Summary

Remove all FormatV1-specific code paths and infrastructure from index reader. System has only written V2 indexes since Phlare v0.1.0, V1 paths were never needed. Follow up to #5155

Details

• Remove FormatV1 constant and version field from Reader struct
• Remove postingsV1 map (V1-only offset table)
• Simplify postings reading to V2-only path with ReadOffsetTable
• Remove version field from Symbols struct, simplify Lookup/ReverseLookup
• Update Version() to return FormatV2 directly
• Remove version checks from Series, SeriesBy, LabelNamesFor, LabelValueFor, Postings, PostingsForLabelMatching

Context

Phlare v0.1.0+ only writes V2 indexes. V1 reader support is legacy code from before standardization. No production systems depend on V1 reading capability.

Test Results

✓ pkg/phlaredb/tsdb/index tests pass (70.2% coverage)
✓ make lint clean
✓ All affected reader methods validated

Risk Assessment

Negligible risk - V1 format never used in production Phlare. No migration needed.

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Note

Medium Risk
Touches core TSDB index reading paths (symbols, postings, series ID offsets) and could break compatibility with any remaining V1 index files, but behavior for V2 should be unchanged and code is largely deletions/simplification.

Overview
Drops V1 index compatibility and makes the reader V2-only. newReader now rejects anything except FormatV2, removes the Reader.version/postingsV1 infrastructure, and always builds postings offsets via ReadOffsetTable.

Simplifies symbols and series offset handling. NewSymbols no longer takes a version, Symbols lookup/reverse-lookup always uses the V2 offset-sampling approach, Reader.Version() is hardcoded to FormatV2, and series lookups (Series, SeriesBy, LabelNamesFor, LabelValueFor) always apply the 16-byte padding offset logic. Tests are updated to match the new NewSymbols signature, and the memdb index package removes its unused FormatV1 constant and related branching.

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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Unused FormatV1 constant left as dead code

Low Severity

The FormatV1 constant was correctly removed from pkg/segmentwriter/memdb/index/index.go, but the equivalent FormatV1 constant in pkg/phlaredb/tsdb/index/index.go (lines 51–52) was left behind even though all its usages in that file were removed by this PR. It's now dead code with zero references inside the package or anywhere else in the repository.

 

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