Add ADRs for insurance pool, NFT invoices, multisig admin; add SDK getLpInvoices

[Samuel1505]

Summary

Closes #555,
closes #556,
closes #557,
closes #558.

Docs — Architecture Decision Records

• Write ADR for insurance pool design #555 — docs/adr/ADR-006-insurance-pool-design.md: documents why the
  insurance pool is a separate contract from invoice_liquidity, the
  accounting-not-custody / flat-coverage-cap scope of the current stub, the
  timelocked coverage/admin-change flow, and the claim_default → pool
  claim integration hook. Flags that the hook is documented in
  docs/insurance-pool-design.md but not yet wired into claim_default.
• Write ADR for NFT invoice representation #556 — docs/adr/adr-007-nft-invoice-representation.md: documents the
  InvoiceNftMetadata data model and the mint/transfer/burn lifecycle design
  in contracts/invoice_liquidity/src/nft.rs. Flags that none of
  mint_invoice_nft, transfer_invoice_nft, or burn_invoice_nft are
  currently called from submit_invoice, fund_invoice, or mark_paid —
  only the read-only queries are wired into the contract's public API today.
• Write ADR for multisig admin #557 — docs/adr/adr-008-multisig-admin.md: documents the M-of-N
  threshold scheme, the 24-hour (MULTISIG_WINDOW_LEDGERS) proposal
  expiration window, and the closed AdminAction enum in
  contracts/invoice_liquidity/src/multisig.rs. Flags — traced via git log
  and MULTISIG_IMPLEMENTATION.md — that the full contract integration
  (initialize_multisig_admin, propose_pause/propose_unpause,
  sign_proposal, execute_proposal) shipped in commit d267e36 but was
  lost from lib.rs in a later merge-conflict resolution (9e94e45); the
  module and its test suite (tests_multisig_admin.rs) exist but are not
  declared in lib.rs and are not reachable from the contract today.
• All three ADRs are added to the index in docs/adr/README.md.

Each ADR documents the design as built, including where the
implementation currently falls short of the original design, so contributors
don't assume more is wired up than actually is.

SDK — getLpInvoices (#558)

• Adds getLpInvoices(server, contractId, lp, page?, pageSize?, networkPassphrase?)
  in sdk/src/methods/lpInvoices.ts, wrapping the contract's
  list_invoices_by_lp(lp, page, page_size) view
  (contracts/invoice_liquidity/src/lib.rs:627).
• page defaults to 0, pageSize defaults to 10 (the contract itself
  clamps page_size to 50). Returns a decoded Invoice[] for the requested
  page (empty past the last page), reusing the existing decodeInvoice
  helper.
• Unlike the existing listInvoicesByLP in queries.ts (which requires the
  caller to construct and pass a sourceAccount), this method follows the
  newer getReputation/getTopPayers convention: a self-contained read-only
  simulation with a G-address validity check, no caller-supplied account
  needed.
• Wired into ILNClient#getLpInvoices and the iln singleton, and exported
  from sdk/src/index.ts.

Test plan

• sdk/src/methods/lpInvoices.test.ts — decoded page of invoices,
  default/custom page/pageSize passed through as u32, empty page, empty
  retval, invalid-address rejection, RPC/simulation errors.
• sdk/src/client.test.ts — iln.getLpInvoices throws before
  iln.configure(...) is called.
• npx vitest run (sdk) — same 13 pre-existing failures across 5 files
  present on main before this change (unrelated: nft.test.ts mock
  account setup, governance.test.ts filter assertion, a
  get_token_decimals address-type test); all new tests pass, total passing
  count increased by 10 with no new failures.
• npx tsc --noEmit (sdk) — same 18 pre-existing error lines on main
  (unrelated: batch.ts, client.ts:389, events/types.ts,
  xdrDecoder.ts:33); no new errors introduced (verified via git stash
  diff against main).
• npx eslint on all touched/added files — clean.
• No positive-path Rust tests were added/changed for the ADR issues,
  since they are documentation-only; the "not wired up" findings recorded in
  the multisig and NFT ADRs were confirmed by grepping lib.rs for the
  relevant module/function names and cross-checking with git log.

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