feat(tree-escrow): dead tree replacement - sponsor can request replanting

[arandomogg]

Summary

Implements dead tree replacement for the single-donor escrow flow in contracts/tree-escrow. When a planted tree is verified dead, the original sponsor can trigger a free replant that is backed by the retained escrow balance — no new deposit is required, exactly as the issue describes.

How it works

The single-donor lifecycle previously was: Funded → Planted (75% released) → Completed (remaining 25% released on survival). This PR inserts a death/replant detour:

Planted ──verify_dead──▶ Dead ──request_replant──▶ Planted (fresh survival clock)

• verify_dead(farmer, death_proof) — admin-only. Attests that a planted tree died. Moves the escrow Planted → Dead and records the death proof. No tokens move; the un-released balance (Tranche 2) stays locked to back the replant. Rejected if the escrow is not Planted, or if nothing remains in escrow to fund a replant.
• request_replant(farmer) — authorized by the original donor (sponsor). Requires status Dead. Restarts the survival cycle for free: status returns to Planted, planted_at resets to now (fresh 6-month clock), survival fields clear, and replant_count increments. No funds are required or moved.
• verify_survival then releases the remainder once the replanted tree survives 6 months — unchanged.

The replant is "free" to the sponsor because the original escrow balance continues to back the outcome; the death/replant cycle can repeat if a replanted tree also dies.

Changes

• New EscrowStatus::Dead variant.
• EscrowRecord gains death_proof: BytesN<32> and replant_count: u32; both deposit paths initialize them.
• New verify_dead and request_replant entrypoints + events (dead, replant).
• Fixes a pre-existing test compile break (BatchSlot literals were missing the gift_recipient field), so the crate's test suite builds again.

Tests

8 new tests; full suite is green (36 passed, 0 failed):

• marking dead retains funds and moves no tokens
• verify_dead rejected before planting and after completion
• replant restarts the survival cycle and costs the sponsor nothing
• replanted tree releases the remainder on survival
• replant resets the 6-month survival clock
• repeated death/replant cycles increment replant_count
• request_replant rejected when the tree is not marked dead

Compatibility

Scoped entirely to tree-escrow. Additive entrypoints and additive struct fields; existing flows (deposit, planting, survival, refund, co-funded, oracle) are unchanged and still pass.

Closes Farm-credit#489

[arandomogg]

Reopening against upstream Farm-credit/stellar-app-os:main.