release: v0.16.0

[octalide]

Cuts v0.16.0, versioning the unreleased work that has accumulated on dev since v0.15.0 so it can sweep to main.

Bumps mach.toml to 0.16.0 and adds the CHANGELOG entry. Minor bump: introduces a new riscv64-linux target (additive), plus darwin and doc fixes — no breaking changes.

Covers:

• riscv64-linux runtime + syscall layer (runtime: riscv64-linux _start + syscall stubs #306) and atomics (sync/atomic: riscv64 arms (blocked on mach RV64A inline-asm support) #308), with a cross-riscv64 qemu CI lane
• darwin runtime symbol exports (runtime/darwin: _rt_argc/_rt_argv/_rt_envp missing #[symbol] annotations — darwin entry fails to link #314) and aarch64 syscall encode fix (system/os/darwin: aarch64 syscall wrappers use 'mov xN, {aN}' (slot-bound source) — fails inline-asm encode #315)
• linux page_size doclint fix (doc: page_size docstring trips the doclint 'documented component' warning #310)

This is the release-prep commit; once on dev it becomes the tip for the dev→main sweep, matching the repo's release pattern (release commit lands on dev, then dev→main, then tag v0.16.0).

Note: "Built with mach 2.9.0" reflects the latest mach release that mach-std CI validates against (linux + cross-riscv64). The darwin cross-build's end-to-end proof is on the mach side against mach dev (carrying #1682) — adjust the version line if you'd rather pin that.