feat(pip-003): Ed25519 checkpoint signing + trust layer architecture

Every Merkle checkpoint is now signed with a locally generated Ed25519
keypair. Signature embedded as C2SP extension line:
sig/ed25519:<pubkey_hex>:<sig_hex>

Trust layers (each strictly additive):
- Merkle: ordering + integrity (RFC 6962)
- Ed25519: identity binding (this commit)
- TSA: time binding (via punkgo-jack, not in kernel)

Changes:
- Add signing.rs: Ed25519 key lifecycle + checkpoint signing (7 tests)
- Integrate signing into audit.rs checkpoint creation
- Add signing_pubkey IPC read kind
- Add audit_tsa_tokens table (forward-compatible, jack-side storage)
- Add trust-layers.svg visual
- Add PIP-003 spec (EN + ZH)
- Rewrite README as landing page (190 → 86 lines)
- Update ARCHITECTURE.md to v0.5.0
- Remove orphaned crates/punkgo-sandbox/
- Bump workspace version 0.4.0 → 0.5.0

138 tests, cargo fmt + clippy clean.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: FeilixX

CHANGELOG.md

@@ -5,6 +5,20 @@ All notable changes to PunkGo Kernel will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [0.5.0] - 2026-03-16
+
+### Added
+
+- **PIP-003: Ed25519 checkpoint signing** — every Merkle checkpoint is now signed with a locally generated Ed25519 keypair. Signature format: `sig/ed25519:<pubkey_hex>:<sig_hex>` in the C2SP checkpoint extension line
+- **Signing key auto-generation** — keypair created on first boot at `{state_dir}/signing_key`, loaded on subsequent starts
+- **`signing_pubkey` read kind** — IPC clients can query the kernel's Ed25519 public key for offline verification
+- **`audit_tsa_tokens` table** — reserved schema for jack-side RFC 3161 TSA timestamp storage (forward-compatible, kernel does not make HTTP calls)
+- **`verify_checkpoint_signature()` and `parse_sig_extension()`** — public functions for third-party offline signature verification
+
+### Dependencies
+
+- Added: `ed25519-dalek` v2 (with `rand_core`), `rand` v0.8, `hex` v0.4
+
 ## [0.4.0] - 2026-03-13
 
 ### Changed

Cargo.lock

@@ -7,7 +7,7 @@ members = [
 resolver = "2"
 
 [workspace.package]
-version = "0.4.0"
+version = "0.5.0"
 edition = "2024"
 authors = ["Felix <feijiu@punkgo.ai>"]
 license = "MIT"
@@ -33,3 +33,6 @@ tempfile = "3.16"
 uuid = { version = "1.21.0", features = ["v4", "serde"] }
 interprocess = { version = "2.4", features = ["tokio"] }
 fs2 = "0.4"
+ed25519-dalek = { version = "2", features = ["rand_core"] }
+rand = "0.8"
+hex = "0.4"