feat: reserve release version-override location pointer

[joshua-temple]

Problem

cascade derives the next version from conventional commits. Some version intent (forcing a pre-release line, pinning an exact release version) cannot be expressed that way. The manifest had no reserved addressing shape for maintainer-committed override files carrying that intent, so adopting it later would mean a non-additive schema change.

Fix

Enrich release: with a reserved location-pointer sub-field rather than introducing a top-level block (the override location is a release-policy knob, and ReleaseConfig already owns release settings):

• ReleaseConfig.VersionOverrides *VersionOverridesConfig (version_overrides, omitempty).
• New VersionOverridesConfig with a single reserved dir pointer (relative, no .. segments). Only the addressing shape is frozen; file format and the fold-into-version-calculation behavior are additive and post-1.0, mapping onto the existing version primitives (bump level and pre-release line) with no parallel enum.
• Structural validation mirrors components.path: reject absolute or ..-bearing dirs, but only when the block is present, so a manifest valid without it is never rejected.
• JSON schema regenerated: releaseConfig gains version_overrides referencing a new closed versionOverridesConfig def (additionalProperties: false). All three on-disk copies stay byte-identical.
• Docs (versioning, configuration) and an e2e scenario added.

Reserved shape only: parse + structural validation, no generator/state/runtime behavior, no schema_version bump.

Verification

• go build ./cmd/... ./internal/..., go test ./... (1512 passed), golangci-lint run ./... (clean).
• e2e module: go build ./... + go vet ./... clean.
• Generator byte-identity test proves a populated version_overrides yields output byte-equal to the absent case (guarded against a vacuous empty comparison). TestPlan_MatchesGeneratedBytes stays green; the generator and version calculation are untouched.
• Schema drift test confirms the three schema copies match.

Closes #178