[SPIKE] zstd dependency choice

[kirich1409]

Context

Evaluate SwiftPM-compatible zstd integrations: zstd-swift, SwiftZstd, or raw C-shim over libzstd. Produces an ADR in docs/adr/0001-zstd-choice.md. New dependency requires explicit user approval before being added to Package.swift.

Acceptance criteria

• ADR written with decision and rationale.
• If a new dependency is proposed, user approval recorded in this issue.

Files to touch (estimate)

best-effort estimate; agent refines during implementation

Implementation notes

n/a

Dependencies

None.

Safety checklist

See .github/ISSUE_TEMPLATE/task.yml — tick applicable invariants in the issue if any destructive path is touched.

Definition of Done

• Acceptance criteria satisfied and verifiable
• Tests added or updated
• No build / lint / test regressions
• Self-review against research-report invariants
• Code review approved