Document and lock the supported library API

[Jason-Morcos]

This is the last of my planned PRs to this repo to share the key findings that allowed me to access my devices!

What changes

• documents the supported module-by-module API for errors, authentication,
  sessions, probes, endpoint discovery, all three CoAP carriers, OwnerPSK, and
  the reusable OCF cache/worker helpers;
• documents the additive 0.1.x compatibility policy, including the existing
  certificate constructor pairs and provider-preferred path;
• records the caller-owned session/threading lifecycle and keeps credential,
  ownership, and Home Assistant policy outside the helper modules;
• expands the public compatibility test to import the exact symbols exercised
  by LocalThings, the reference bridge, and the downstream integration;
• asserts that the root packages remain intentionally free of duplicate
  re-exports;
• syntax-checks every Python example in the README; and
• moves the reference bridge's fmt_code import to its canonical coap
  module without changing behavior.

There is no new runtime facade and no transport behavior change in this
commit.

Validation

• 717 tests pass on Python 3.14
• all README Python blocks compile
• wheel and sdist build, contain the expected tracked files, and pass the
  distribution checker
• share-safety check passes against Add opt-in HVR-only peer cleanup #77's head

Merge order

This is stacked on #77, which is stacked on #76. Please merge in this order:

1. Add authenticated server identity discovery #76 — authenticated first-use server identity
2. Add opt-in HVR-only peer cleanup #77 — opt-in HVR-only peer cleanup
3. this PR — PY-18 import contract and release documentation

The GitHub diff currently includes the earlier commits. After #76 and #77
merge, this PR reduces to commit 0660924. #74 is independent and can merge at
any point.