Add a max investor count guard per invoice to prevent unbounded settlement iteration cost

[Chucks1093]

Summary

The settlement function iterates over all investor positions to distribute returns. An unbounded investor count could exhaust the Soroban transaction budget. A maximum investor count per invoice enforced in the invest function would cap the settlement cost within safe limits.

Scope

• Add set_max_investors(count: u32) admin function to configure the cap (default 500)
• Track the investor count per invoice in a persistent counter keyed by invoice_id
• In invest, panic with MaxInvestorsReached if adding a new investor would exceed the cap
• Increment the counter only for first-time investors on a given invoice
• Emit a max_investors_updated event when the cap is changed

Acceptance Criteria

• First investment from a new wallet increments the investor counter
• Subsequent investments from the same wallet do not increment the counter
• New investor on a full invoice panics with MaxInvestorsReached
• Non-admin set_max_investors panics with Unauthorized
• max_investors_updated event emitted on change

ETA: 24 hours

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Coordinate on Telegram

[Outlaw1299]

@Outlaw1299 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

I’ll like to work on this issue, please assign

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[The-Big-Danny]

@The-Big-Danny has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

hello maintainer, i'd love to work on this issue! please assign me

ℹ️ Repo Maintainers: To accept this application, review their application or assign @The-Big-Danny to this issue.

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