feat: wire QueryExpr algebra pipeline into handle_plan — optimizer, SketchAllocator, PlanSummary (#109 follow-up)

[zzylol]

Summary

• parse_query_expr added to query_parser/mod.rs — dispatches to parse_promql_expr / parse_sql_expr which emit QueryExpr directly from the AST (no lossy SketchExpr round-trip), preserving HistogramQuantile, PromQLSubquery, vector-binary-op matching, Sort+Limit, Join, and SetOp
• handle_plan now runs the full algebra pipeline when query_string is present: parse_query_expr → QueryOptimizer → SketchAllocator → PlanSummary; the plan_summary key is included in the /api/v1/plan JSON response (stage assignments, bandwidth saved, budget demotions, per-node rationale)
• Compilation fixes for the new direct-emit path: TokenType(u8) match via .id(), AggItem alias conflict (AlgAggItem), CrossJoin(_) tuple variant, SetOperator::Minus coverage, duplicate SourceSpec import, wc move-before-use in handle_plan

Partially addresses #109 (parser bridge gap + PlanSummary; coverage gap tasks remain open)

New response fields (/api/v1/plan when query_string is supplied)

{
  "plan_summary": {
    "bandwidth_saved_bytes_per_sec": 12345.6,
    "agent_memory_bytes": 65536.0,
    "backend_memory_bytes": 0.0,
    "has_budget_demotion": false,
    "node_annotations": [
      { "node_kind": "SketchAgg", "stage": "agent", "mode": "sketch",
        "rationale": "DDSketch quantile at agent", "memory_bytes": 65536.0, "bytes_per_sec": 800.0 },
      ...
    ]
  }
}

plan_summary is null when query_string is absent or the parser fails (non-fatal warning logged).

Test plan

• cargo build — clean (0 errors, 13 expected warnings)
• cargo test — 285 passed, 0 failed
• Integration: POST /api/v1/plan with query_string: "quantile_over_time(0.99, latency[5m])" returns plan_summary with at least one agent-stage sketch node
• Integration: POST /api/v1/plan without query_string returns plan_summary: null (no regression)

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