fp-hw-survey capture: Ampere Altra / AWS Graviton (arm64/Linux)

Capture native FP behavior on a **server ARM64** part — Ampere Altra and/or AWS Graviton2/3/4 (`aarch64` / Linux) — with `crates/fp-hw-survey`.

### Steps
```sh
cargo build --release -p fp-hw-survey
./target/release/fp-hw-survey info # confirm CPU brand + features (rpres/afp probed via std runtime detector on Linux)
./target/release/fp-hw-survey selftest # must pass
./target/release/fp-hw-survey capture --label -linux
```

### What to attach to this issue
- The `info` output (CPU brand + detected features).
- The `selftest` result (must be clean — `capture` refuses to write otherwise).
- The **one-line NDJSON header** from the capture (durable provenance: cpu / os / features / date).
- The `capture-*.ndjson` file itself (attachment or link).

### Notes
- On Linux the std `is_aarch64_feature_detected!` path is active, so `rpres`/`afp` are detected where present — these are the only knobs that can change a single-precision estimate value.
- **Microsoft Cobalt 100 (Neoverse-derived) is explicitly out of scope** for now per rook DESIGN-NOTES D-37; do not file it under this issue.

Part of the FP hardware survey fleet-capture campaign.