feat: hard-tier PECL extensions on aarch64 (phase 2 C2b)

[phramz]

Summary

Closes the top-50 PECL × arm64 matrix. Direct mirror of B2 on aarch64.

• Extends abi_matrix.arch on imagick, mongodb, swoole, grpc to include aarch64.
• Adds 5 multi-ext-hard4-arm64-<NN> harness fixtures (one per PHP version; -81 omits swoole).

Pure data slice. No Go/builder/workflow changes. Inherits all scaffolding from C1 + C2a.

Carry-over exclude (arch-independent)

Extension	Exclude	Reason
swoole	{ php: "8.1" }	PECL metadata: <min>8.2.0</min>

4 × 5 − 1 = 19 new arm64 ext bundles.

Known risks (playbook from B2 / C2a)

1. imagick IM6 on arm64 — Ubuntu noble ships libmagickwand-6.q16-7t64/libmagickcore-6.q16-7t64 for both archs. Expected clean; if not, exclude: { arch: "aarch64" } + issue.
2. grpc bundled-source on arm64 — grpc upstream has first-class arm64 since ≥1.50. If PECL bundled-source fails, fall back to system libgrpc-dev/libprotobuf-dev.
3. swoole configure on arm64 — default configure only; swoole upstream supports arm64 natively.
4. mongodb OpenSSL on arm64 — libssl3t64/libsasl2-2 arch-uniform.

Wall-clock dominated by 5 × arm64 grpc + 5 × x86_64 grpc spec-hash-refresh ≈ 45 min.

Test plan

• CI: planner emits 19 new arm64 ext cells + 19 x86_64 refresh cells.
• CI: arm64 ext builds succeed.
• CI: lockfile auto-commits 19 new entries.
• CI: 65 harness fixtures (60 existing + 5 new) pass with no new allowlist entries.
• CI: `make check` passes.

Spec: `docs/superpowers/specs/2026-04-21-phase2-aarch64-c2b-design.md`