feat(sdk-coin-sol): rewrite fee payer for versioned custom transactions

[ralph-bitgo]

What

• In CustomInstructionBuilder.fromVersionedTransactionData(), when feePayer() is set and differs from staticAccountKeys[0], the versioned message is rewritten so the fee payer becomes static account 0 (the fee payer of a versioned transaction is simply staticAccountKeys[0] as supplied, so .feePayer() previously had no effect):
  • fee payer not among the static keys → inserted at index 0 as a writable signer and numRequiredSignatures is incremented;
  • fee payer present elsewhere → moved to index 0: numRequiredSignatures is incremented if it was a non-signer, and the read-only count of the section it left is decremented (numReadonlySignedAccounts for a read-only signer, numReadonlyUnsignedAccounts for a read-only non-signer);
  • every static and lookup-table index is remapped with one full old → new map, so instructions keep targeting the same accounts;
  • a fee payer that is an address lookup table account is rejected (a table account can never sign).
• The rewrite runs before injectNonceAdvanceInstruction(), so when the fee payer is also the nonce authority it is already a signer and carries a single signature, with AdvanceNonceAccount still instruction 0.
• Adds EnterpriseFeePayerParams { feePayer: string } to src/lib/iface.ts as the named params type for sponsored builds.
• Documents on fromVersionedTransactionData() that wallet-platform must NOT set a fee payer for a versioned transaction a third party has already signed (the rewrite invalidates existing signatures); wallet-platform builds those wallet-pays.

Why

• Enterprises must fund SOL wallets one by one today just to pay network fees and rent (CHALO-983). The fee-payer design lets one enterprise fee address sponsor its wallets' transactions, but caller-built versioned transactions (customTx, WalletConnect) cannot be sponsored without this rewrite, because a versioned message's fee payer is whatever the caller put in staticAccountKeys[0]. The rewrite only happens when .feePayer() is called, so no bytes change for today's callers.
• Verified compatible with CHALO-1651's open PR (fix(sdk-coin-sol): fix three bugs in injectNonceAdvanceInstruction #9846, the injectNonceAdvanceInstruction() fixes): merged its branch into this one on a throwaway branch and the full module suite passes (783 tests, including all fee-payer tests). The tests deliberately assert via indexOf-computed indexes instead of pinning nonce-injection header details, so fix(sdk-coin-sol): fix three bugs in injectNonceAdvanceInstruction #9846's fixes do not break them. The only merge conflict is textual in the test file (both PRs append a describe block at the same anchor); keeping both blocks resolves it.

Stack

This PR is part 2 of 2 in a stack. Review and merge in order:

1. feat(sdk-coin-sol): add addFeePayerSignature() #9848 — addFeePayerSignature() (CHALO-996 dependency surface; base: master)
2. feat(sdk-coin-sol): rewrite fee payer for versioned custom transactions #9849 — fee-payer rewrite for versioned custom transactions (base: CHALO-1652-fee-payer-rewrite-versioned-tx-pt1) ← you are here

Test plan

• npx mocha test/unit/transactionBuilder/customInstructionBuilder.ts: fee payer inserted at account 0 with correct header and remapped indexes (message compiles and round-trips); a real Jupiter swap proves lookup-table instructions still target the same accounts after the +1 shift; non-signer / read-only non-signer / read-only signer moves with the matching header adjustments and no duplicate key
• durable-nonce case: fee payer = nonce authority → one signature from it, AdvanceNonceAccount is instruction 0 with the authority at account 0
• addFeePayerSignature() produces byte-identical output to the native sign flow (slot 0); non-account-0 keys are rejected
• without feePayer() (and with feePayer() equal to the existing account 0) bytes are identical to a direct MessageV0 construction of the caller's data
• guards: fee payer that is an address lookup table account and invalid fee payer addresses are rejected
• npm run lint and tsc --build clean in modules/sdk-coin-sol
• Full module unit suite green (773 passing)

Ticket: CHALO-1652

[linear-code]

CHALO-1652