[Bench #478] Add peak RSS, allocation-count and copy-volume parity to canonical benchmarks

[godofecht]

Parent: #478

Runtime alone is not enough for the claim that Flow beats Python. Add memory evidence beside every representative canonical and scaled workload.

Measure process peak RSS, heap allocation count/bytes where instrumentable, persistent model size, temporary workspace size and explicit matrix/vector copy volume. Separate cold process/import memory from steady estimator execution. Record whether BLAS/native libraries allocate internally when observable.

Acceptance: same-machine Flow/sklearn memory rows for all canonical estimator families; machine-readable fields and Pages plots; regressions can be gated directionally; a runtime win with materially worse memory remains an explicit yellow/red dimension rather than being hidden.

[godofecht]

Worked through the memory-evidence contract and archived the full result in godofecht/OpenProjects as ml/phase-local-live-memory-certificates.md with an adjacent Flow witness.

The key recommendation is to report an exact phase-local live-memory certificate rather than trying to infer estimator memory from RSS alone. For a complete allocation/release trace, define live bytes L(t) and compute P=max_t L(t). With E live bytes at phase entry and X at exit, report H=P-E as additional live headroom and R=X-E as retained model growth, alongside cumulative allocated bytes, explicit copy payload, and category-specific peaks.

Two exact counterexamples matter for #491. First, category peaks are not additive: entering with 100 persistent bytes, then using 300 workspace bytes, freeing them, then using a 250-byte copy gives a total peak of 400 even though workspace+copy category peaks sum to 550. Second, allocation volume does not determine peak memory: two traces can both allocate 630 bytes and retain the same 80 bytes while peaking at 400 vs 650 solely because workspace and copy lifetimes overlap.

RSS should remain a process-level guardrail, but allocator page retention/reuse means RSS delta cannot identify workspace or copy pressure. Suggested machine-readable operation vector: (entry_live, peak_live, exit_live, headroom, retained_delta, allocated_bytes, copy_payload_bytes, peak_workspace, peak_copy) plus RSS separately. This makes zero-copy, arena/reuse, and workspace optimizations accountable to the memory dimension they are supposed to move.