Feat: mentorship, skill verification, charity split & tax strategies (#884, #885, #886, #887)

[olivia-brien]

Summary

This PR adds four new standalone Soroban contracts to the workspace. Each follows the existing invoice-escrow layout (lib.rs, errors.rs,
types.rs, test.rs), uses typed contracterror errors, emits events for every state change, and comes with its own unit tests.

Changes

#884 – On-chain creator mentorship program (contracts/mentorship)

• Mentors register with an expertise, a revenue share (≤ 30%) and a mentee capacity; they can update their terms, and the admin can suspend them.
• Programs follow the lifecycle request → accept/decline → active → complete/cancel, with a fixed term in ledgers.
• During a program, mentors log sessions and mentees route the mentor's revenue share of their earnings through share_earnings.
• After completion, mentees can rate the mentor (1–5), and mentor stats are tracked on-chain.
• Events: mentor / reg, updated, suspend, requested, accepted, declined, session, shared, completed, cancelled, rated

#885 – Recipient skill verification (contracts/skill-verification)

• Recipients submit skill claims backed by an evidence hash, and can withdraw them while pending.
• The admin appoints verifiers per skill. Verifiers approve claims at a level (1–5) with an expiry, or reject them with a reason hash. Self-verification
  is blocked.
• Attestations can be revoked by the issuing verifier or the admin.
• Views for gating recipients: is_skill_verified, get_skill_level, meets_requirement, all_meet_requirement.
• Events: skill / verifier, claimed, withdrawn, verified, rejected, revoked

#886 – Invoice charity donation split (contracts/charity-split)

• The admin maintains a registry of vetted charities and can activate or deactivate them.
• An invoice donates a configurable share (1–5000 bps) of each recipient's payout to its chosen charity; rounding dust also goes to the charity.
• Invoices release automatically once fully funded. Payers can make extra donations, and creators can cancel an invoice, after which payers claim
  refunds.
• Per-charity and per-donor donation totals are tracked, and preview_split shows payouts before release.
• Events: charity / reg, status, created, paid, released, donated, cancelled, refunded

#887 – Creator tax optimization strategies (contracts/tax-strategy)

• Each income payment is split into an optional pre-tax deduction, a tax reserve, and the creator's net amount.
• The reserve can be computed three ways: a flat rate, creator-defined marginal brackets, or admin-published bracket templates.
• Brackets apply to cumulative income within a configurable tax period; when a period ends it rolls over and its totals are archived.
• estimate and estimate_config let creators compare strategies before switching.
• The contract never holds funds; all transfers go directly to their destinations.
• Events: tax / tmpl_set, tmpl_del, strategy, removed, income, period

Other

• Added all four crates to the workspace members in Cargo.toml.
• Existing contracts are unchanged.

Closes #884
Closes #885
Closes #886
Closes #887

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