load every manifest into one component-major in-memory model

[joshua-temple]

Problem / why

cascade should run promote / version / release / hotfix over ONE internal model regardless of
how the manifest is written. The first step is a pure-internal component-major model: a
manifest with no components: block loads as a single synthesized default component, all in
memory, with ZERO change to the on-disk shape, the generated output, or any behavior. This is
invisible to users and de-risks the later multi-component work by exercising the component
dimension behind the existing single-component surface.

This is the foundation root issue. It carries no schema change and no expectation risk because
nothing observable changes.

Scope

This PR does:

• Introduce a typed in-memory component-major container that the rest of the code reads
  through, for example a *ManifestState wrapping Components map[string]map[string]*EnvState
  (component name to environment name to EnvState), plus accessors (get-or-create
  components[name][env], ComponentNames()).
• On load, synthesize a single default component from the top-level
  builds / deploys / environments / triggers / tag_prefix and re-key the environment
  state under components.default.<env> IN MEMORY only.
• Keep the on-disk shape, the serializer output, and all generated workflows byte-identical:
  for a single-component manifest, serialization still emits state.<env> (the collapse form,
  fully specified in F3; this PR may stub the collapse so round-trip stays byte-stable).
• Re-point the in-memory reads in internal/promote, internal/hotfix, internal/rollback,
  internal/orchestrate, and internal/version to go through the default component via the
  accessors.

This PR does NOT:

• Add any components: parsing (F2) or the dual serializer's expand form (F3).
• Change any on-disk bytes or generated output.
• Add a components: user surface (V1).

Files / areas touched

• internal/config/types.go - the in-memory container and accessors.
• internal/config/parse.go - synthesize default on load.
• internal/promote/, internal/hotfix/, internal/rollback/, internal/orchestrate/,
  internal/version/ - read state through the default component.

Tests and coverage required

• Unit tests on the accessors (get-or-create nested entry, component enumeration).
• A round-trip test: a single-component manifest loads into the component-major model and
  serializes back byte-identically to its input.
• The existing promote / hotfix / rollback / version / orchestrate unit tests pass unchanged.

Acceptance criteria

• All state reads go through the component dimension via the default component.
• A single-component manifest round-trips byte-identically (load to component-major, serialize
  back to state.<env>).
• No generated workflow output changes.

Dependencies

None within the epic. It is the root foundation issue.

[joshua-temple]

#278 was closed as superseded by this milestone. The reserved component fields it proposed to
remove become the foundation for the component-major model built here (#280, #281, #282), so
they are evolved into the schema-2 shape rather than deleted and re-added.

[joshua-temple]

Done in PR #507: manifests load into one component-major in-memory model (TrunkConfig.HasComponents / ResolveComponent in internal/config/components.go), with the single-component case as the implicit default. Shipped in v0.12.0 and exercised by the whole-fleet-green run (fleet-e2e 28963078743). The PR referenced this with Refs rather than Closes, so it stayed open.