Publish TypeScript client bindings for integrators

[joelpeace48-cell]

Problem

An integrating gateway currently has to hand-roll contract invocation from the
raw interface. That is friction at exactly the point where adoption is decided,
and every integrator will build a slightly different, slightly wrong version of
the same code.

What to do

• Generate bindings with stellar contract bindings typescript as part of the
  release process, so they cannot drift from the deployed interface.
• Publish to npm under an org scope, versioned in step with contract releases.
• Layer a thin ergonomic wrapper over the generated output: typed errors rather
  than raw codes, and network presets carrying the deployed addresses.
• Include a runnable example that attests and reads back, in under twenty lines.
• Document the compatibility matrix: which SDK version works with which
  deployed contract version.

Acceptance criteria

• Bindings generated in CI, not by hand
• Published to npm with versioning tied to contract releases
• Ergonomic wrapper with typed errors and network presets
• Runnable quickstart example
• Compatibility matrix documented

Notes

The frontend and backend should consume this package too. If it is good enough
for our own code it is probably good enough for an integrator, and dogfooding
is what keeps it honest.