docs(vesting): add VestingWallet API reference and correct released/init event schemas

[Dinma179]

Documentation follow-up for #407, #408, #409 and #410. The contract work for all four landed in #425 (merged); these issues were reopened afterwards, so this PR documents what is now in main — and fixes two real errors in the published event schema.

New: docs/vesting-api.md

A single reference for the vesting-wallet contract that a client or indexer
actually needs:

• the linear-with-cliff formula, explicitly noting vesting runs from
  start_ledger (not the cliff) in ledger sequence numbers, not wall-clock time;
• every read entry point, plus the guidance that get_vesting_overview() is the
  one call a dashboard should use for schedule and progress;
• VestingOverview and VestingSchedule field tables, including why
  releasable_amount saturates instead of going negative after
  emergency_withdraw;
• every write entry point with its authorization requirement;
• release_all, including why it takes no beneficiary argument — the
  beneficiary is read from storage and is the account that must authorize, and a
  parameter would either be ignored or let a third party force a release at a
  moment the beneficiary did not choose;
• all four VestingWallet events and the deployment front-running caveat.

Fixed: docs/events.md

The registry states that "off-chain indexers rely on these definitions", so two
errors in it are real defects rather than cosmetic gaps:

1. released was documented with two payload fields. The contract has always
   published three — (beneficiary, releasable, released + releasable). An indexer
   built from the table would mis-parse the tuple and could not track cumulative
   progress, which is the outcome [bug] vesting-wallet: release doesn't emit event #408 exists to prevent. The third field is now
   documented, with a note to track released_total rather than summing per-call
   amounts.
2. The init event was undocumented. initialize emits it, and it was
   absent from the registry entirely — an indexer had no schema for the event
   that marks a schedule's creation.

Both corrections are documentation-only; no contract behaviour changed, and the
registry remains the source of truth alongside the new reference.

Closes #407
Closes #408
Closes #409
Closes #410

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