docs: add research spikes and design docs for issues #49, #60, #114, …

[shadow-ghost706]

docs: research spikes & design docs for cross-chain proof, multi-bond,
multi-sig (#49 #60 #114 #124)

What this PR does

Delivers all four documentation tasks from the research/design backlog.
No contract code is changed — these are design docs and research spikes
that unblock future implementation work.

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Changes

File	Issue	Summary
docs/49-cross-chain-proof-spike.md	#49	Surveys Acurast, DIA,
Chainlink CCIP, Wormhole, LayerZero, and ZK light clients for source-chain
proof delivery to Stellar. Recommends Wormhole generic messaging (only
production-ready option with a live Stellar integration and no per-message
fee).
docs/60-multi-bond-token-design.md	#60	Full data-model redesign to
support multiple bond tokens with per-token accounting. Covers new storage
keys, updated API signatures, lazy migration for existing SolverRecord
entries, and a complete test matrix.
docs/114-multisig-admin-design.md	#114	Evaluates native Stellar
multi-sig vs in-contract logic vs external guardian contract. Recommends
using a native Stellar multi-sig account as Admin — zero contract
changes required. Includes CLI setup script, key-holder policy, signer
rotation ceremony, and an optional PauseGuardian split for fast incident
response.
docs/124-proof-verification-interface.md	#124	Concrete interface
design for ProofRegistry (new contract), changes to fill_intent
(opt-in require_proof flag), VortexDeposit.sol sketch for source
chains, VAA payload layout, and a 3-phase rollout plan that introduces
proof gating without breaking existing fills.

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Closes

Closes #49
Closes #60
Closes #114
Closes #124

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Testing

These are documentation-only changes. No build or test changes are
included.
Implementation PRs for each design will reference this PR and include the
corresponding contract changes and test coverage.

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Reviewer notes

• Design the Stellar oracle/messaging interface for source-chain proof verification #124 depends on the Wormhole recommendation from Research spike: cross-chain proof verification via Stellar oracle/messaging infra #49 — read
  49-cross-chain-proof-spike.md first.
• Add support for multiple bond tokens with per-token accounting #60 is a standalone data-model change with no dependency on the other
  three.
• Evaluate and design multi-sig admin control #114 is an operational recommendation; the only optional code change is
  the PauseGuardian split described in §6.

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