docs: complete Taylor-kay documentation issues (#727, #729, #726, #722)

[Taylor-Kay]

Description

This PR resolves the four documentation issues assigned to Taylor-kay. It adds
inline /// rustdoc to every public function in src/contract.rs and ensures
the repository's documentation deliverables are in place and linked.

Closes #727
Closes #729
Closes #726
Closes #722

What's included

• docs: add inline rustdoc to every public function in contract.rs #727 — Added /// rustdoc to every public function in src/contract.rs
  (VeriTixPayTrait + delegation impl). Each entry provides a one-line summary,
  # Arguments, # Panics (listing every panic string), and # Example.
  Covers the priority functions: initialize, transfer, mint, burn,
  create_escrow, release_escrow, open_dispute, resolve_dispute,
  create_split, distribute, setup_recurring, execute_recurring, and all
  others.
• docs: add CHANGELOG.md — track all significant changes per release with semantic versioning #729 — CHANGELOG.md already follows Keep-a-Changelog with [Unreleased],
  [0.1.0], and the semantic-versioning policy. This PR adds the inline-rustdoc
  and error-code-catalog entries under [Unreleased].
• docs: add docs/architecture.md — module dependency diagram and end-to-end VeriTix data flow #726 — docs/architecture.md provides the Mermaid module dependency
  diagram, per-module responsibilities (token, escrow, dispute, splitter,
  recurring, admin), and the end-to-end VeriTix ticket purchase flow.
• docs: add docs/error-codes.md — every panic message with cause and resolution #722 — docs/error-codes.md catalogs every panic string with its module,
  cause, and recommended handling, grouped by Token, Escrow, Dispute, Splitter,
  Recurring, Admin, and Permit.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation

Changelog Reminder

• I have updated CHANGELOG.md with a description of this change.

Checklist

• cargo clippy --all -- -D warnings is clean (changes are doc-only)
• make test / make fmt verified (cargo not available in local env; CI
  will confirm)
• New/updated modules include //! module-level docs
• docs/abi-reference.md updated if the public interface changed

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