Add envelope widening fixture (#74)

[sehkone]

Summary

• Add a test-support payload fixture that rewrites envelope lengths while retaining the private footer layout.
• Document sparse-file construction and cover bounded and unbounded reader behavior.
• Keep the fixture unavailable in default-feature builds and leave production writers and readers unchanged.

Closes #74

Test plan

• Build a signed container, widen both envelope blocks, and confirm bounded reading reports wrong-length metadata without reading either block.
• Confirm an unbounded package reader attempts the widened envelope and fails instead of accepting empty blocks.
• Confirm the widened container still exposes its unverified manifest.
• Confirm a dependent-style test can write the compact fixture sparsely with test-support enabled.
• Run cargo build, cargo fmt -- --check --config group_imports=StdExternalCrate, both required Clippy configurations, both test configurations, and RUSTDOCFLAGS='-D warnings' cargo doc --no-deps.

[sehkone]

[Reviewer Round 1]

Approved — no blocking findings.

The fixture at src/payload.rs keeps the footer layout private, is correctly feature-gated, and constructs the compact prefix/footer form needed for sparse writing. Its tests exercise both bounded non-reading and unbounded envelope access, while the integration test verifies dependent-style public use.

PR #75 correctly uses Closes #74 and includes a test plan.

[sehkone]

[Review Verdict Round 1: APPROVED]

[sehkone]

Suggested squash commit

Title

Add envelope widening fixture

Body

Keep malformed-envelope construction inside deploy-core so dependent
metadata tests can cover bounded reads without duplicating private footer
layout. Validate widening inputs and provide compact bytes that consumers
can write sparsely.

Closes #74