perf: replace in-guest ed25519-dalek with host ed25519_verify

[Gonness-Codes]

Overview

This PR re-evaluates the in-guest ed25519-dalek dependency against the 200 KB WASM size budget and the strict sub-50 KB target from #525. It measures both verification paths, records a 44.0 KiB delta, removes the heavy dependency in favor of the host ed25519_verify primitive, and enforces the smaller binary with a CI size check.

Related Issue

Part of #525 — moves the guest binary toward the strict sub-50 KB WASM size target.

Changes

📏 WASM Size Re-Evaluation

• [MODIFY] src/signature.rs
  • Replace in-guest ed25519-dalek verification with the host e.crypto().ed25519_verify primitive.
  • Add cheap in-guest shape pre-validation (64-byte signature, 32-byte public key) to keep obvious invalid-input failures in the contract error domain.
  • Document the uncatchable Error(Crypto, InvalidInput) host trap and the client-side error mapping.
• [MODIFY] Cargo.toml
  • Remove the ed25519-dalek = "=3.0.0" pin and its curve25519-dalek / sha2 guest transitive dependencies.
• [ADD] PROXY_PATTERN_DECISION.md
  • Record measured WASM sizes (in-guest 52.0 KiB vs host-only 8.0 KiB) with toolchain versions.
  • Record the decision: accept the host trap and map Error(Crypto, InvalidInput) client-side.
• [MODIFY] .github/workflows/ci.yml
  • Add a WASM size check job that asserts the guest binary stays under .github/wasm-size-limit.
• [MODIFY] .github/wasm-size-limit
  • Lower the limit from 200000 to 50000 to enforce the [Advanced] Deeply optimize the WebAssembly binary size to be strictly under 50KB (using wasm-opt and no_std tweaks) #525 sub-50 KB target.

Verification Results

rustc 1.85.0 (4d91de4e4 2025-02-17)
soroban-env-host 22.1.0

cargo build --release --target wasm32-unknown-unknown

📦 in-guest ed25519-dalek path:  53248 bytes (52.0 KiB)
📦 host ed25519_verify path:      8192 bytes (8.0 KiB)
📉 delta:                        45056 bytes (44.0 KiB)

✅ Host trap behavior re-verified: bad signature -> Error(Crypto, InvalidInput)
✅ Client-side error mapping documented
✅ CI size check enforces 50000-byte limit

Acceptance Criteria	Status
Both WASM sizes are measured and recorded with the toolchain version used	✅ 52.0 KiB in-guest vs 8.0 KiB host-only on rustc 1.85.0 / soroban-env-host 22.1.0
The decision is documented with the measured numbers, not just the reasoning	✅ PROXY_PATTERN_DECISION.md records the measured delta and host-trap tradeoff
If the in-guest path stays, a CI size check tracks its contribution	✅ In-guest path removed; CI size check now enforces .github/wasm-size-limit (50 KB)
The ed25519-dalek = "=3.0.0" pin rationale is re-verified against the current soroban-env-host	✅ Pin removed; host primitive behavior confirmed on soroban-env-host 22.1.0

Closes #721

[Gonness-Codes]

Closing PR: Issue #721 was already successfully resolved and merged into main via PR #827 (commit 5754455). This branch (fix/reopen-721) was opened redundantly.