feat(spec): add immutable node profile contracts

[mdheller]

Summary

Adds the first SourceOS machine-readable immutable-node contract tranche.

Added

Schemas:

• schemas/ImmutableNodeProfile.json
• schemas/HostCapabilityPlacement.json
• schemas/NodeStateSchema.json

Examples:

• examples/immutablenodeprofile.json
• examples/hostcapabilityplacement.json
• examples/nodestateschema.json

Validation/docs:

• tools/validate_immutable_node_examples.py
• docs/contract-additions/immutable-node-profiles.md

Why

The platform standards repo now has ADR-070 for the generic immutable host placement grammar and an ADR-071 PR clarifying the SourceOS/Socios boundary.

This PR turns the SourceOS side into typed contracts so downstream repos can render, validate, and emit evidence around immutable-node profiles without making Socios a mandatory base OS dependency.

Boundary

• SourceOS immutable node profiles are base substrate contracts.
• Optional Socios capability packs are modeled as optional references only.
• substrate.sociosRequired is constrained to false.
• This does not replace BootReleaseSet.
• This does not implement boot execution, host mutation, or runtime rendering.

Validation

Focused validation command added:

python3 tools/validate_immutable_node_examples.py

Upstream state

This branch is additive but currently behind main by recent upstream movement. It should be replayed onto current main before merge.

Follow-on work

1. Wire the new validator into Makefile.
2. Index the schemas in schemas/README.md and ARCHITECTURE.md.
3. Add Agent Machine renderer/validator support.
4. Add sourceosctl immutable-node plan|validate|inspect in sourceos-devtools.
5. Register cross-repo topology edges in sociosphere.

[mdheller]

Superseded by #95.

Reason: this PR was the first immutable-node contract tranche, but the replacement PR reframes the contracts explicitly as node / agent-runtime substrate work rather than desktop-owned work and was replayed onto a newer base branch.

Please review #95 instead.