Latency telemetry: preserve real histogram samples

[proerror77]

Change contract

Export each real latency sample to Prometheus exactly once at its production recording boundary, without reconstructing distributions from rolling summaries.

Closes #397

Out of scope

• p99.9 retention policy (Latency telemetry: add evidence-grade p99.9 retention and sufficiency #398)
• Timestamp capture semantics (CEX market data: make exchange and local timestamp semantics type-safe #395)
• WS transport provenance (CEX market data: correct WebSocket receive-boundary provenance #396)
• Execution-path span coverage (CEX execution: instrument intent-to-write, ACK, and private-report spans #399)
• SLO threshold changes

Dependency or merge order

Stacked on #438. Merge order: #438, then this PR. After #438 merges, retarget this PR to main before merge.

Focused validation

• rustfmt --edition 2021 --config skip_children=true --check infra-services/core/metrics/src/lib.rs apps/live/src/helpers/metrics.rs market-core/engine/src/dataflow/ingestion.rs market-core/engine/src/lib.rs market-core/runtime/src/system_builder.rs
• cargo test -p hft-infra-metrics
• cargo test -p hft-engine
• cargo test -p hft-engine --features metrics
• cargo clippy -p hft-engine --features metrics -- -D warnings
• cargo check -p hft-live --features metrics
• cargo check -p hft-runtime --features metrics
• Spec and standards re-review: no remaining findings

The focused skewed-sample test asserts count, sum, bucket shape, and no replay after periodic gauge sync.

Rollout / rollback impact

Metrics-correctness change only. No execution enablement, risk-policy change, or deployment action. Live remains disabled. Roll back by reverting commit 6160b150a9a75db404dead258f917ddfcf71082f.

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📥 Commits

Reviewing files that changed from the base of the PR and between 7aee974 and 417219c.

📒 Files selected for processing (6)

• rust_hft/apps/live/src/helpers/metrics.rs
• rust_hft/infra-services/core/metrics/src/lib.rs
• rust_hft/market-core/engine/src/dataflow/ingestion.rs
• rust_hft/market-core/engine/src/execution_worker.rs
• rust_hft/market-core/engine/src/lib.rs
• rust_hft/market-core/runtime/src/system_builder.rs

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[chatgpt-codex-connector]

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