docs(nvpair-errors): add normative service specification (#129)

## Description

Adds `services/nvpair-errors/spec.md`, the normative specification for
the error store and its cross-node sync, derived from the Go source and
its tests and following the structure of the existing specs.

Peer sync is described as it runs today: every address a peer publishes
is kept in the peer's own order, and the one to push to is confirmed by
connecting and remembered until a transport failure retires it. Errors
are relayed to the desktop client and `nvpair-tui`, and each
`nvpair-proxy` engine facade is a producer. Open items record that the
list does not survive a restart, that an entry with an empty `nodeId`
cannot be cleared, and that the installer opens UDP 5353 for a binary
that opens no multicast socket.

## Release intent

<!-- pair-release-intent:v1 -->
### Changelog title
n/a

### Changelog body
n/a

### Bumps
- services: none
- nvpair-cluster-manager: none
- nvpair-engine-manager: none
- nvpair-errors: none
- nvpair-job-scheduler: none
- nvpair-manual-nodes: none
- nvpair-node-info: none
- nvpair-node-scanner: none
- nvpair-node-settings: none
- nvpair-proxy: none
- nvpair-tui: none
- nvpair-ui-broker: none
- nvpair-workload-manager: none
<!-- /pair-release-intent:v1 -->

## Scope

Documentation only: one new file. No Go, desktop, installer, or version
changes. The open items above are recorded, not fixed.

## Validation

- `node scripts/spdx-headers.mjs` passes.
- Every behavioral statement was checked against the source on
`develop`.

## Risk

None at runtime. `spec.md` is normative, so please flag any statement
that does not match the code.

## Checklist

- [x] I have read the [Contributing
Guidelines](https://github.com/NVIDIA/Personal-AI-Router/blob/main/CONTRIBUTING.md).
- [x] Every commit is signed off (`git commit -s`), certifying the
[Developer Certificate of Origin](https://developercertificate.org/).
- [ ] New or existing tests cover the change. (Not applicable:
documentation only.)
- [x] Relevant documentation is updated.
- [x] I checked the diff, changed filenames, and commit messages for
credentials, private data, internal URLs, internal issue identifiers,
and generated artifacts.
- [x] I recorded the validation commands and results above.
- [x] I declared version bumps in the release-intent block above.
`services/versions.json` is written by automation — do not edit it by
hand.

---------

Signed-off-by: Terve <ntervalon@nvidia.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: Noah-Tervalon-Nvidia