docs(examples): document the allocate_cbtc DvP example

[sosaucily]

Summary

examples/README.md documents every example except allocate_cbtc, which landed with the DvP allocation client in #59. Someone reading the README to find out what the library can do wouldn't learn that DvP settlement is supported at all.

Found while updating the public CBTC developer docs to cover DvP — the docs site had no mention of it either, and this README was the natural place to have discovered it.

What's added

An "Allocate CBTC (DvP)" section, placed with the other transfer-related examples (after Cancel Pending Transfers), covering:

• How to run it and which env vars it needs
• How an allocation differs from send_cbtc — an allocation is settled by a third party (the settlement executor) across all legs atomically, rather than accepted by the receiver
• The allocateBefore / settleBefore ordering constraint
• The unwind paths (withdraw, cancel) and the executor's execute_transfer
• That no Minter credential is required, since allocations move existing CBTC rather than minting or burning it

Plus an "For the allocate_cbtc example" block in the Environment Variables section:

Var		Default
LIB_TEST_RECEIVER_PARTY_ID	required	—
EXECUTOR_PARTY_ID	required	—
ALLOCATE_AMOUNT	optional	0.1
SETTLEMENT_REF_ID	optional	cbtc-dvp-example

Verified against the env::var calls in examples/allocate_cbtc.rs rather than assumed.

Note: four other examples are also undocumented

While checking, these are in examples/ but absent from the README:

• check_dars
• check_withdraw_requests
• list_deposit_addresses
• list_withdraw_accounts

Left out to keep this PR to one thing. Happy to add them in a follow-up if useful — say the word.

Testing

Docs-only change, no code touched. Confirmed every *.rs in examples/ except the four listed above now has a matching cargo run --example <name> entry in the README.

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