feat(memory): adopt mimalloc allocator

[randomvariable]

Purpose

Adopt mimalloc for Rust-owned heap allocations while preserving native allocator relief and process-level telemetry.

Stacked on cortexkit#283. Review only commits 692c81e2 and 36f6c668 until cortexkit#283 merges.

Changes

• install mimalloc as the Rust global allocator in production and allocation-counted tests
• derive Rust allocator requested, committed, and retained-byte telemetry from mimalloc statistics
• run mi_collect(true) after idle eviction and from the detached allocator-maintenance thread
• retain malloc_trim(0) on glibc and malloc_zone_pressure_relief on macOS for native libraries that bypass Rust GlobalAlloc
• preserve the fleet health memory field names and byte units
• document the allocator coverage boundary
• add a realistic multi-root daemon comparison protocol for cold build, steady serving, idle eviction, and post-relief residency

Coverage boundary

mimalloc statistics cover Rust-owned heap allocations. SQLite, tree-sitter, ONNX Runtime, and other native libraries can still allocate through the platform allocator. RSS, macOS physical footprint, SQLite bytes, and subsystem estimates remain independent process checks.

Verification

• allocator relief contracts passed
• fleet health payload compatibility contracts passed
• cargo check -p agent-file-tools --all-targets
• cargo fmt --all -- --check
• git diff --check

The realistic multi-root benchmark protocol is committed without results. Both allocator arms still require paired representative-host runs before an allocator performance claim.

Scope

This PR changes allocator ownership and reclamation only. Explicit subsystem budgets, charge-before-build admission, and cache eviction policy remain separate work.

Contribution

AI-assisted implementation and verification with OpenAI Codex through Oh My Pi. The author reviewed the changes and test evidence.