fix(ktuner): shmall target uses runtime page size

[jfeng18]

What

eval_shmall hardcoded a 4096-byte page size when converting RAM to pages. kernel.shmall is measured in pages, and page size is architecture-dependent (4 KiB on x86_64, up to 64 KiB on arm64). On 64K-page kernels the recommended value was ~16x too high.

Fix

• Read the page size at runtime via sysconf(_SC_PAGESIZE) (fallback 4096).
• Extract a pure shmall_target_pages(mem_gb, page_size) that guards page_size == 0.
• Replace the previous vacuous if let test with a discriminating one: asserts the 4K result equals the 64K result x16, and page_size 0 -> 0.

Testing

• Only unit-tested: 1.89.0 cargo fmt --check / clippy --all-targets -- -D warnings / cargo test all pass (192 tests).
• Mutation-tested: reverting to hardcoded 4096 fails the scaling assert; removing the zero-guard panics; flipping /2 fails — the test is discriminating.
• Not E2E-verified on a real 64K-page arm64 kernel (dev host is x86_64/4K); the page-size scaling is covered by the pure-function test injecting 65536.