Personal-systems asset inventory maintained as a register

## Summary

Maintain a plain register of every host, device, and service in the
personal estate — the unglamorous spine the diagrams reference. Not
a diagram: a table.

## Motivation

Every view (network/physical map, DFD, threat model) references the
same underlying facts — what hosts exist, their addresses, what runs
where, how they're monitored and backed up. Keeping that in one
authoritative register means the diagrams cite a single source
instead of each re-encoding (and drifting from) the facts. It's also
the fastest artifact to reach for during an investigation.

## Scope

Output is `docs/`.

- A register (table) with, per entry: hostname, IP/address, role,
OS/firmware and version, physical location, owner, criticality,
data classification, monitoring coverage, backup status.
- Cover the known estate: TrueNAS appliance, HAOS device, TP-Link
Deco mesh, off-site NanoPi-NEO3, plus the cloud/identity surface
(GCP project, Cloudflare zone, GitHub) at a useful granularity.
- Include a credential sub-register: every Active operator/API token
(Cloudflare, GCP, GitHub) with its scope and its documented
consumer, distinguishing Terraform-managed tokens from
hand-created ones. The point is to surface orphans — a token that
maps to no documented consumer (as the #128 audit found with the
hand-created Cloudflare `Edit zone DNS` template token) should be
visible as such, so a periodic token evaluation can't miss it.
- Keep it lightweight and maintainable — the same update-on-change
contract as the C4 model applies.

## Acceptance criteria

- [ ] Register exists under `docs/` with the columns above
- [ ] Covers all known home and off-site devices plus the
cloud/identity surface
- [ ] Credential sub-register maps each Active operator/API token to a
documented consumer and flags orphans
- [ ] Diátaxis classification: reference

## Additional context

Spine data for the network/physical map and the backup/DR topology.
Format/strategy recorded in the documentation-strategy ADR.
Structural model: #84. Credential-register need surfaced by the #128
Cloudflare token audit.