feat: independently versioned components in a single manifest

[joshua-temple]

cascade assumes one versioned unit per repository: a single set of builds and deploys, a
single environment-level version, and a single latest-release record. Repositories that
ship several independently versioned components in one tree cannot model that today, and
retrofitting it after the schema freeze would require a breaking change to the manifest
top level and to the recorded state.

Scope for the v1.0.0 milestone is narrow: reserve only the top-level component schema
shape before the freeze. The generator, state, and runtime behavior are explicitly out of
scope here and land in a later (post-1.0) change. The goal is that a future release can add
per-component versioning without removing or retyping any field that shipped at v1.0.0.

Proposed approach:

• Define and document a reserved top-level shape for addressable components (for example a
  components map) and a reserved per-component slot in recorded state and in the latest
  release record. This shape reservation is the only part that must land before the v1.0.0
  freeze.
• Apply parse-and-structural validation for the reserved shape only; no emit behavior yet.
• Keep the single-component manifest the canonical default; the reserved shape must be
  absent-by-default and non-breaking for every existing manifest.

Acceptance criteria:

• The reserved component shape parses and validates structurally, with no generator output
  attached to it.
• Every existing single-component manifest continues to parse, validate, and generate
  identical output.
• docs/versioning records the reservation so the later implementation is additive.
• A unit test asserts the reserved shape round-trips and that omitting it changes nothing.