feat(key-wallet): add missing DIP-15 derivation constants to dip9

[shumkov]

Issue being fixed or feature implemented

Every other DIP-9-family feature index (identities 5' with its sub-features, coinjoin 4', dashpay 15', platform payment 17') is defined in key_wallet::dip9, but two DIP-15 derivation families were never added:

• the auto-accept feature 16' — m/9'/coin_type'/16'/expiry', the expiry-bounded bearer keys behind the DashPay auto-accept QR (dapk);
• the contactInfo hardened children — root/65536'/index' (encToUserId) and root/65537'/index' (privateData) under the owner's identity-authentication path.

As a result, downstream consumers in dashpay/platform carry their own copies today: rs-platform-wallet (crypto/auto_accept.rs local const, crypto/contact_info.rs named consts) and rs-sdk-ffi's resolver signer (raw 16/65536/65537 literals — it cannot import platform-wallet). Duplicated magic numbers for canonical derivation constants are exactly what dip9 exists to prevent (noticed while fixing the same issue for the invitation constants in dashpay/platform#4041).

What was done?

Added to key-wallet/src/dip9.rs, beside their siblings, each with a doc comment describing the derivation family:

• FEATURE_PURPOSE_DASHPAY_AUTO_ACCEPT: u32 = 16
• DASHPAY_CONTACT_INFO_ENC_TO_USER_ID_CHILD: u32 = 1 << 16
• DASHPAY_CONTACT_INFO_PRIVATE_DATA_CHILD: u32 = (1 << 16) + 1

No behavior change — documented constants only. The dashpay/platform call sites will switch to them on their next rust-dashcore bump.

How Has This Been Tested?

cargo check -p key-wallet + cargo fmt clean; additive constants only, no runtime paths touched.

Breaking Changes

None (additive).

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests (n/a — documented constants)
• I have made corresponding changes to the documentation if needed

🤖 Generated with Claude Code

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Review details

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📥 Commits

Reviewing files that changed from the base of the PR and between 2045cd2 and 6ab1ac0.

📒 Files selected for processing (1)

• key-wallet/src/dip9.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/dip9-auto-accept-feature-const

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.33%. Comparing base (2045cd2) to head (6ab1ac0).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #886      +/-   ##
==========================================
- Coverage   74.37%   74.33%   -0.04%     
==========================================
  Files         327      327              
  Lines       74747    74747              
==========================================
- Hits        55590    55563      -27     
- Misses      19157    19184      +27     

Flag	Coverage Δ
core	77.26% <ø> (ø)
ffi	50.31% <ø> (ø)
rpc	20.00% <ø> (ø)
spv	90.96% <ø> (-0.16%)	⬇️
wallet	74.40% <ø> (ø)

Files with missing lines	Coverage Δ
key-wallet/src/dip9.rs	6.38% <ø> (ø)

... and 5 files with indirect coverage changes