fix(cargo-anvil): shorten Windows coverage paths

[martintmk]

Summary

• use target/c as cargo-llvm-cov's private build directory on Windows unless the caller already set CARGO_LLVM_COV_TARGET_DIR
• keep the existing per-feature report split while shortening every --object path passed to llvm-cov export
• add recipe contract coverage and update the generated Anvil files and design documentation

Motivation

Oxidizer PR microsoft/oxidizer#722 still fails after the per-feature split because one llvm-cov export command is 34,152 characters, exceeding the Windows CreateProcess limit. Replacing target/llvm-cov-target with target/c in that captured command reduces it to approximately 28,088 characters while preserving the same object set and coverage semantics.

This is intentionally a targeted mitigation for cargo-llvm-cov's current one-argument-per-object behavior. A response-file implementation in cargo-llvm-cov would be the structural upstream fix.

Validation

• focused recipe contract tests for the compact target directory
• cargo-anvil clippy with all targets and features
• generated artifact dry-run reports no drift

[martintmk]

replaced by #166