feat(standards): BurnNote bon builder

[VAIBHAVJINDAL3012]

Summary

Converts BurnNote from a unit marker + BurnNote::create factory into a struct built with a bon typestate builder (BurnNote::builder()), mirroring the merged P2idNote builder. One of three per-note PRs splitting the standard-note builder work (Swap / Burn / Mint).

• BurnNote::builder() produces a BurnNote that converts into a Note via Note::from (always public, tagged for the faucet).
• Migrated the 6 faucet burn-note test sites.

Builder-only change; no MASM or on-chain behaviour changes.

Verification

• cargo clippy -p miden-standards -p miden-testing --all-targets --all-features -- -D warnings - clean
• New note::burn::tests::builder_builds_public_burn_note unit test; the faucet burn suites pass and re-run in CI here.

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• Create PR with unit tests
• Commit unit tests in branch vaibhav/burn-note-builder

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