feat: reserve components schema shape for per-component versioning

[joshua-temple]

Problem

schema_version is a monotonic int. Adding a per-component versioning dimension after the v1.0.0 freeze would force a breaking schema bump. Closes #176.

Fix

Reserve the shape only, before the freeze, as a purely additive change:

• Optional components map on the trunk config, per-env state, and latest-release state (all omitempty, absent by default).
• New ComponentConfig/ComponentState/ComponentReleaseState types.
• Structural validation (validateComponents) rejects unsafe component names and ../absolute paths.
• No generator output and no behavior attached. schema_version stays at 1; implementation is reserved for later.
• versioning doc records the reservation so future work is additive.

Verification

• New tests assert: the reserved shape parses and validates structurally; it round-trips through parse and marshal; omitting it changes nothing; existing manifests still generate byte-identical output (the generate golden suite is untouched).
• go build ./... && go test ./... && golangci-lint run ./... all green.
• Independent review: APPROVE.