[A-EDev]

Summary

A payload's body must now be exactly one gzip member, as flow-sync #102 specifies, so libraries that read gzip framing differently can't take different data from one signed object.

• The body must start with 1f 8b 08 00, so FLG is 0. MTIME, XFL and OS aren't checked.
• Nothing may follow the member. This uses bufread::GzDecoder rather than read::GzDecoder, because the latter's internal buffer swallows what follows the member, so into_inner couldn't see it.
• GzDecoder still checks the CRC-32 and ISIZE.
• The 64 MiB stop stays as the read's limit. A read that stops there leaves the trailer unread, so the leftover check refuses it, and the separate length comparison had nothing left to catch.
• This device's own payloads still round trip, and the limit is tested on both sides.
• Vectors from flow-sync 345a836, byte for byte. The fixtures README is that commit's whole file, which also brings its pairing line up to date.

Follows flow-sync #102.

Related issue

None.

Change type

• Feature

Validation

Run on macOS arm64.

• pnpm test: not run, no frontend file changed
• pnpm build: not run, no frontend file changed
• cargo fmt --manifest-path src-tauri/Cargo.toml --all -- --check: exit 0
• cargo clippy --all-targets --all-features in src-tauri: exit 0, no warnings in the files this changes
• cargo test --no-fail-fast --all-targets --all-features in src-tauri: 1287 passed, 0 failed, 9 ignored
• I smoke-tested the affected desktop behavior. Nothing ran in a real window.

Before the change, payloads failed on the new negatives. Checked by breaking the code on purpose, one change at a time, with sync2_vectors:

Change	Tests that failed
A flag is allowed	1
No check of the start	1
Leftover bytes are allowed	2
read::GzDecoder, whose buffer hides leftovers	1
The read has no limit	1
The limit is one higher	1
The limit is one lower	1

Risk and compatibility

• No format or database change. Every encoder writes FLG 0 and one member; this PR tests only this app's encoder.

• Not run against Android or a server under this rule.

• Overlaps other open PRs on the fixtures README and the changelog.

• No new secrets, private data, telemetry, or broad Tauri permissions were introduced.

• User-facing documentation was updated where needed. (spec/crypto.md in flow-sync [Bug]: Severe UI lag in theatre mode (ResizeObserver layout loop) + main window opens floating/above on Hyprland #102)

• Dependency and lockfile changes are intentional and limited to this PR. (none)

• Breaking changes and upgrade steps are clearly documented. (none for honest clients)

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