add mobench support to ProveKit main

[dcbuild3r]

Summary

• add main-based mobench fixtures for passport, OPRF, and P-256 bigcurve
• run BrowserStack triad profile across Vivo Y21, Pixel 7, S24 and iOS fallback triad
• fix Android prove timing so fixture clone/setup is outside the measured prove span
• use jemalloc as the Android fallback allocator under the native C FFI allocator
• harden Android incomplete-fixture reporting with timeout/build/kill-evidence fields

Android mobench fix note

The previous failing run was https://github.com/worldfnd/provekit/actions/runs/26007468825. The missing Vivo Y21 monolithic cell did not recover a BrowserStack session payload or summary.json; available artifacts only show the BrowserStack fetch timeout after 7200s for build c943753d95fd0b34f5775aa0a3bc6ff58cbcc3ca. I grepped the recovered Android artifacts and job log for lowmemorykiller, Process * was killed, oom_reaper, SIGKILL, and abnormal signal text; there was no hit because BrowserStack did not return the killed session logs for that cell.

Before/failure memory from that run:

• Vivo Y21 OPRF: 328 MB process peak, 10.061s
• Vivo Y21 fragmented age check: 1187 MB process peak, 65.342s
• Vivo Y21 monolithic age check: no row; BrowserStack timeout/no summary
• S24 OPRF: 426 MB process peak, 2.809s

What changed:

• Android and iOS now use the same semantic boundary for prove benches: per-iteration fixture setup happens before the measured closure, and profile_phase("prove") wraps only the prover entry point. Previously prepared.clone() ran inside profile_phase("prove"), so clone cost and peak memory were charged to proving.
• Android native C FFI fallback allocation now routes through jemalloc instead of Bionic malloc when no host callback/mmap allocator is active.
• If BrowserStack returns no summary, failure.json now records attempts, fetch timeout seconds, build id, and any LMK/OOM/SIGKILL lines recovered from attempt/device logs.

After numbers: pending the fresh BrowserStack rerun on this commit.

Validation

• cargo fmt --all
• cargo test -p bench-mobile --lib
• cargo test -p bench-mobile --test examples_smoke
• cargo test -p bench-mobile --test passport_smoke
• cargo check -p provekit-ffi --target aarch64-linux-android with NDK 26.1 aarch64-linux-android34-clang
• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/mobile-bench-reusable.yml")'
• git diff --check