docs: record design decisions and closed questions as ADRs

[hpmaxi]

Closes#8.

Records Decisions 1–6 plus the four open questions as eight Nygard-format ADRs in docs/adr/.

AC mapping

• Deposit gating → ADR-0002: receiver-only; exits never gated nor pausable.
• LP counter → ADR-0008: removed; reconstructed off-chain from events if needed.
• Role names → ADR-0003: runtime Symbol::new, full names, all granted at deploy.
• Licence → ADR-0007: Apache-2.0 (LICENSE + NOTICE already on main).
• ADRs committed in Nygard format → this PR.

Beyond the original AC (resolved since the issue was filed)

• ADR-0003 also closes the Role Manager question with the spike evidence from Does Role Manager detect a stellar-access contract? #7: read/audit only; execution via the admin signing harness (grant/revoke argument-order bug reported upstream as [adapter-stellar] grant_role/revoke_role argument order is pinned to stellar-contracts v0.5.x — breaks against >= v0.6.0 OpenZeppelin/openzeppelin-adapters#72; no Stellar multisig execution in the adapter).
• ADR-0001 records explicit-leaf composition as the design (not a workaround awaiting stellar-contracts#560).
• ADR-0006 pre-authorizes the bounded fork-minimal fallback for stellar-contracts#674.