fix(batch-processor): implement real batched token transfers

[johnbiliyaminu-bit]

Replace the arithmetic-only stub with a genuine batch transfer contract.

Previous implementation:

• Took only Vec amounts, no addresses or token
• Summed amounts with checked_add and returned the total
• Contained dead state-version race-condition checks (always snapshot+1 under Soroban's single-transaction, non-reentrant model)
• Contained dead re-entrancy guard (redundant given Soroban's execution model)
• Emitted no transfers despite the contract name and BatchTooLarge error

New implementation:

• Signature: process_batch(env, funder, token, recipients, amounts)
• funder.require_auth() called before any token movement
• Validates lengths match, batch <= 100, all amounts > 0
• Accumulates total in one pass, pulls a single lump sum from funder, then fans out to each recipient individually
• Error enum: LengthMismatch=1, BatchTooLarge=2, InvalidAmount=3, ArithmeticOverflow=4

Closes #248, #104, #246

What does this PR do?

Type of change

• Bug fix
• New feature
• Refactor (no behaviour change)
• Test coverage
• Documentation
• Security fix

Related issue

Closes #

Changes

File	Change

Checklist

• cargo fmt --all — no diff
• cargo clippy --all-targets -- -D warnings — zero warnings
• cargo test --all — all tests pass
• New public functions have tests covering the happy path and each error variant
• All arithmetic uses checked_* methods
• require_auth() called before any state mutation in modified functions
• State mutations happen before cross-contract calls (token transfers)
• Events emitted for all external state changes
• CHANGELOG.md updated under [Unreleased]
• docs/architecture.md updated if design changed
• ADR written if a significant design decision was made

Security notes

Testing notes

closes #361

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