feat: configurable iOS jetsam memory limits

[cjchanh]

What changed

• make iPhone/iPad worker layer budgeting configurable via CAKE_MOBILE_LAYER_BUDGET_MB and CAKE_MOBILE_RESERVE_PCT
• keep existing Apple desktop unified-memory behavior unchanged
• add mobile library / KMP bridge hooks so the shared app can set those values before starting a worker
• add a focused sharding test for the default iPad/iPhone path

Why

Cake currently hardcodes a 1536 MiB mobile layer budget. That is safe as a default, but it makes the iOS worker memory policy inflexible for devices and deployment modes that can safely use a different budget.

This change keeps the current defaults intact and only adds configuration for the iPhone/iPad path.

Impact

• default behavior is unchanged
• iOS worker hosts can opt into higher or lower budgets without patching Cake core
• the mobile app can pass those settings through the existing shared worker startup flow

Validation

• RUSTC_WRAPPER="" cargo test -p cake-core --lib --test unit
• RUSTC_WRAPPER="" cargo clippy -p cake-core -p cake-cli --features metal -- -D warnings
• RUSTC_WRAPPER="" cargo build --release --features metal
• shared KMP compile: :shared:compileKotlinMetadata, :shared:compileKotlinIosArm64, :shared:compileKotlinIosSimulatorArm64

Notes

• Android shared-module compilation was not run in this clean worktree because the local Android SDK path is not configured there.
• scripts/deploy-ios.sh and broader mobile telemetry/UI work were intentionally left out of this PR.