docs: state the layer openblox occupies and the rule that bounds it

[Lutherwaves]

The non-goals were already written down. What was missing is why the list has
that shape.

ARCHITECTURE.md listed five things openblox does not do; README.md listed
four. Read cold, each item looks like a gap someone might later fill, so every
feature request gets to argue against one bullet on its own merits — and
individually, most of those arguments are reasonable. That is the shape a scope
erodes in.

The rule

How a sandbox is isolated is openblox's problem.
Which sandbox runs where is yours.

Isolation is the runtime, the egress policy, the filesystem, the user, the
capabilities, the resource caps and the lifetime bounds. Placement is
scheduling, queueing, tenancy, metering, fleet membership and snapshots.

Every existing non-goal is that one rule applied, rather than a separate
opinion. Stating it lets a request be declined by pointing at it, without
relitigating the scope each time.

Where openblox sits

Also missing was the positive claim. openblox is the layer below a sandbox
platform, not a smaller one — the absence of a control plane is what makes it
composable, not something a user has to work around. The comparison is
libvirt, not OpenStack: consumed by platforms, outlived the platforms, never
became one.

The two-tier guarantee, said once, plainly

The strongest claim in the README was in its fourteenth paragraph. It now sits
next to the defaults it qualifies, and distinguishes two things that were
running together:

• In the library, your code chooses. The defaults are safe and every
  relaxation is explicit and greppable at the call site — weakening is
  visible.
• Through openbloxd, the choice is not the caller's at all. Profiles live
  in the daemon's config and no request can reach them; a caller names a
  profile, never an image, runtime, user, egress policy or resource cap —
  weakening is unreachable.

The library cannot claim the second, since WithImage takes caller input by
design. Saying so is what makes the daemon's claim credible.

Changes

File
README.md	new "Where this sits"; "What it is not" now opens by naming the rule it applies
ARCHITECTURE.md	new "The rule" above Non-goals, defining isolation vs placement term by term
CONTRIBUTING.md	new "Scope" as the first section — a PR crossing the rule is declined on that basis, so open an issue first
docs/index.md	mirrors the README section for the site

Documentation only. No code, no behaviour, no API. docs: cuts no release.

Summary by CodeRabbit

• Documentation
  • Clarified openblox’s role as an isolation layer beneath broader sandbox platforms.
  • Documented the boundary between supported isolation capabilities and excluded placement features such as scheduling, tenancy, metering, and snapshots.
  • Explained the two levels of isolation guarantees provided by library defaults and openbloxd profiles.
  • Updated architecture and contribution guidance to reinforce project scope and clarify when features should be built externally.

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• CONTRIBUTING.md

📝 Walkthrough

Walkthrough

The documentation defines openblox as an isolation layer. It separates isolation concerns from placement concerns across the architecture guide, contribution guide, README, and documentation index.

Changes

Scope boundary

Layer / File(s)	Summary
Architecture rule ARCHITECTURE.md	Defines the isolation-versus-placement rule, applies it to non-goals, and updates the guidance for declined features.
Contribution scope CONTRIBUTING.md	Lists included isolation concerns and excluded placement concerns for contributors.
User-facing scope documentation README.md, docs/index.md	Explains openblox’s position below a sandbox platform, its restrictive defaults, excluded placement features, and comparison with libvirt.

Priority: ⬇️ Low

Merge Risk: 🔵 Low · up to d1d68

Clarify the API-server exclusion before merging so contributors are not discouraged from valid broker changes.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the documentation changes that define openblox’s architectural layer and scope boundary.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[Lutherwaves]

Rebased onto main — the branch was a month behind and README.md had moved under it. Two conflicts, resolved in favour of main's newer text, keeping this PR's actual contribution:

• ## Where this sits (the layer diagram, the rule, the libvirt comparison) lands intact, now after ## In production: run openbloxd rather than after ## Why, which is where main's structure has room for it.
• ## Secure by default → dropped in favour of main's ## Restrictive by default. main renamed the section and tightened the table since this branch was cut — noexec scratch, no swap, captured output, commands killed at timeout — plus a THREAT_MODEL.md paragraph. Restoring this branch's older, looser version of that table would have been a regression.
• The two-tier guarantee paragraph merged cleanly and kept its intended placement, next to the defaults it qualifies.
• ## What it is not → dropped in favour of main's ## When not to use it, whose five bullets are more concrete. This PR's framing ("none of these are gaps waiting to be filled…") is preserved as the closing paragraph.

One deliberate rewrite: the sentence "The first three follow from the rule above — they are placement, not isolation. The fourth is a trade made on purpose." counted a four-bullet list that no longer exists. main's list has five bullets in a different order, so copying it across would have made a false claim about which bullet is which. It is re-anchored generically instead — same point, no stale numbering.

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Actionable comments posted: 1

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• 🪄 Fix CodeRabbit comments on this PR

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minimal, and validate.

Inline comments:
In `@CONTRIBUTING.md`:
- Line 12: Update the listed exclusions in CONTRIBUTING.md to qualify “an API
server” as a platform/control-plane API server, preserving the existing
exclusion scope while allowing the documented broker API exposed by openbloxd.

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[Lutherwaves]

Good catch from the review, and valid on the merits: cmd/openbloxd is an API server, so listing "an API server" among the placement-side exclusions contradicted the repo one paragraph after stating the rule. Qualified to "a control-plane API server" — the excluded thing is the platform layer above openblox, not the broker API openblox already ships. Thread resolved.