fix: canonicalize Ed25519 algorithm casing

[GsCommand]

Motivation

• Ensure runtime-core emits and verifies the canonical CLAS signature algorithm string Ed25519 so runtime does not need to normalize before verification.

Description

• Change the canonical algorithm constant to SIGNATURE_ALG = "Ed25519" in src/crypto.ts and make signing functions emit metadata.proof.signature.alg via that constant.
• Update verification logic in src/compat.ts and src/receipt.ts to treat Ed25519 as canonical while temporarily accepting legacy lowercase ed25519 by normalizing it internally before comparison.
• Broaden proof type declarations to allow the legacy lowercase value for compatibility and update doc comments to document canonical Ed25519 and the temporary acceptance rule.
• Update README.md to reference the canonical Ed25519, and add/update tests in test/compat.test.ts and test/receipt.test.ts to assert canonical emission, verification without caller normalization, legacy lowercase acceptance, and rejection of unsupported algorithms; files changed: README.md, src/crypto.ts, src/compat.ts, src/receipt.ts, test/compat.test.ts, test/receipt.test.ts; final SIGNATURE_ALG value is "Ed25519"; commit 02561debf528b4b38b6804cc9e65d6b68f0a42cc.

Testing

• Ran npm install, npm run build, npm test, and npm run typecheck, and all commands completed successfully with npm test reporting 72 tests passed and 0 failures.
• Tests added/updated verify that signing emits signature.alg === "Ed25519", that verifyCommandLayerReceipt() and verifyReceipt() accept canonical Ed25519 without caller-side normalization, that legacy lowercase ed25519 remains accepted for compatibility, and that unsupported signature algorithms fail as expected, and all these tests passed.

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Codex Task