test(symbolizer): add symbol-ref tree integration tests and docs

[jake-kramer]

Part of a series: symbol-aware tree references (7 of 8)

Full context: #5317 (PR 1). The final functional PR: end-to-end integration tests + docs.

1. symbolizer: expose address-level Resolve API (feat(symbolizer): expose address-level symbol resolution as a public API #5317)
2. proto: SymbolRefTable + tree fields (feat(query): add SymbolRefTable and symbol_refs tree fields to the query API #5318)
3. model/symbolref: intern table + merge rebasing (feat(model): add symbolref package for symbol-aware tree references #5321)
4. model/symbolref: grouping + rebuild (feat(model): add unresolved-binary grouping and tree rebuild to symbolref #5322)
5. symdb resolver output (feat(symdb): add symbol-ref tree resolver output #5332) · querybackend mode + aggregation (feat(querybackend): produce and aggregate symbol-ref tree reports #5333)
6. frontend orchestration behind a default-off per-tenant flag (feat(frontend): resolve symbol-ref trees after the final merge #5334)
7. integration tests + docs ← this PR
8. symdb: compaction preserves line-less locations (fix(symdb): preserve line-less locations when rewriting partitions without functions #5337) — stacked at the bottom of the series, merges first

Based on #5334.

What

Integration coverage with symbolizer.symbol-ref-trees-enabled=true, each mapped to a fix:

• Queryless {} symbolization via the tenant-wide index (the scenario that motivated grafana/pyroscope-squad#487).
• Span-filtered tree query symbolizes correctly.
• Mixed symbolized + unsymbolized datasets in one query.
• Unknown build ID -> binary!0xaddr fallback, end to end.

Fixtures push unsymbolized profiles through the real ingest path (authentic __unsymbolized__ label), using the common single-mapping shape. Docs: the new flag, resolve timeout, queryless-now-works, and the fallback rendering.

Testing

The four symbol-ref integration tests pass; the existing symbolization test still passes; build/vet/gofmt clean.

[jake-kramer]

This change is part of the following stack:

• fix(symdb): preserve line-less locations when rewriting partitions without functions #5337
  • feat(symbolizer): expose address-level symbol resolution as a public API #5317
    • feat(query): add SymbolRefTable and symbol_refs tree fields to the query API #5318
      • feat(model): add symbolref package for symbol-aware tree references #5321
        • feat(model): add unresolved-binary grouping and tree rebuild to symbolref #5322
          • feat(symdb): add symbol-ref tree resolver output #5332
            • feat(querybackend): produce and aggregate symbol-ref tree reports #5333
              • feat(frontend): resolve symbol-ref trees after the final merge #5334
                • test(symbolizer): add symbol-ref tree integration tests and docs #5335 ◀
                  • test(symbolizer): comparative benchmark, symbol-ref vs pprof-detour #5336

_{Change managed by git-spice.}