Stellar: adapt smart account docs to latest stellar-contracts

[brozorec]

Adapts the Stellar Smart Account docs to the breaking changes on stellar-contracts main (checked at d3f7df8), mainly #868 (auth payload digest) and #866 (soroban-sdk v28, CreateContractExternalRef), plus #848, #759, #819 and #867 for the policy and example updates.

Stacked on #245. Retarget to main once #244 and #245 are merged.

Two commits:

1. Smart account pages adapted to the breaking changes.
2. A fix for the Context Rules example, which did not compile even before this change.

The digest change landed after v0.8.0-rc.3 and is not in a tagged release yet (latest is v0.7.2). Until v0.8.0 ships, these pages describe main rather than what a v0.7.x user runs.

Breaking changes covered

Signed message (authorization-flow.mdx, signers-and-verifiers.mdx, smart-account.mdx)

• Signers now commit to AuthDigestPreimage { account, signature_payload, context_rule_ids } instead of sha256(signature_payload || context_rule_ids.to_xdr()). External signers sign sha256(preimage.to_xdr()). Delegated signers authorize require_auth_for_args((preimage,)), so they sign the struct itself instead of a 32-byte digest.
• New AuthDigestPreimage section in Authorization Flow. Updated the sequence diagram, the delegated and external signer steps, both __check_auth snippets and the Auth Digest Computation steps.
• Added a callout on the v0.7.x → v0.8.0 change that links to the upstream migration guide. Also documented the new auth_digest(preimage) view and added a stellar_xdr encoding of the preimage.

Delegated signer client flow (signers-and-verifiers.mdx)

• The TypeScript example now builds the preimage as an ScVal::Map and passes it as the only argument of the delegated signer's __check_auth entry. The re-simulated JSON shows the map.
• Pre-existing errors fixed along the way: the old example passed hash(HashIdPreimage), which was already wrong for v0.7.x, and it pointed __check_auth at the target contract instead of the smart account. The smart account address now comes from the simulated entry.
• Added a callout for the failure mode (Error(Auth, InvalidAction) with a trap inside __check_auth, not a SmartAccountError). Linked the upstream auth_entries reference test and stellar/smart-account-kit.
• The #transaction-simulation-behavior anchor is unchanged. The upstream README links to it.

Context rules (context-rules.mdx, authorization-flow.mdx)

• New CreateContractExternalRef(Address, String) context type (Protocol 28, CAP-85). Added a callout: such a rule trusts the owner contract, and matching follows how the executable is referenced, so a CreateContract(hash) rule doesn't cover an external-ref deployment of the same hash.

Policies (policies.mdx)

• Weighted threshold: new "Required Operation Ordering" section. When raising, call set_signer_weight before set_threshold; when lowering, call set_threshold before reducing weights. A wrong order reverts with InvalidThreshold (3211). The caveats and the signer-management callouts link to it.
• Spending limit: zero-amount transfers are always allowed, and history is capped at 815 entries (HistoryCapacityExceeded beyond that).
• Pre-existing errors fixed along the way: the window is period_ledgers (in ledgers, not seconds), and the policy only permits transfer calls.

Other pre-existing fixes

• batch_add_signer is not a SmartAccount trait method. It's a helper function that performs no authorization, so the docs now show a contract wrapping it with require_auth, as the multisig example does.
• The overview now links the multisig example, the new factory example and smart-account-kit.

Context Rules example (context-rules.mdx, second commit)

• add_context_rule takes the context type, name, signers and policies by reference. The example passed them by value.
• Added the missing semicolon on the use and the missing Signer import.
• 1_year / 7_days / 12_hours are not valid Rust literals, and current_ledger was undefined. Replaced them with e.ledger().sequence() and a DAY_IN_LEDGERS constant.
• bls_verifier and ed25519_verifier are each used for two signers, so the first use is now cloned. Address is not Copy, so the second use failed as a moved value.

Verification

• Checked every API claim against stellar-contracts main: smart_account/{mod,storage}.rs, the policy modules, the multisig example and test/auth_entries.rs. That test confirms that a delegated entry carrying the digest bytes instead of the preimage struct fails.
• All 5 pages compile with the site's MDX + GFM pipeline, and every internal anchor link resolves.
• The Context Rules example was extracted verbatim, wrapped in a function whose parameters stand in for the placeholder variables, and passed cargo check against stellar-accounts on main. Negative control: the previous version fails with 13 errors.
• Not done: the other Rust snippets on these pages are partial __check_auth excerpts and were not cargo checked. The TypeScript example was not run. pnpm run build was not run.