Move split-timestamp operations behind Concerts

Parent

#158

What to build

Move split-timestamp reads, desired updates, and reset behavior behind explicit Concerts methods. Preserve validation, source-duration coverage rules, media dependencies, and split triggering while returning precise outcomes and resulting Concert State.

Acceptance criteria

• Timestamp reads, desired updates, and reset operations use explicit Concerts methods.
• Validation and coverage failures are typed expected rejections with latest state when available.
• Successful changes return resulting Concert State without an adapter-level re-query.
• Unknown required media facts prevent unsafe timestamp actions rather than being treated as absent.
• Existing HTMX editor routes and interaction behavior remain unchanged.
• Interface-level tests cover automatic, user-defined, reset, invalid, incomplete-coverage, missing-media, and retry outcomes.
• Adapter tests cover request parsing and response translation only.
• Documentation and the Change Record include timestamp state transitions.

Blocked by

• #162

Read the parent issue #158 and blocker #162 first, then trace the current split-timestamp operations, Concerts interface, adapters, and existing HTMX editor routes. Use the interface-level tests to cover the listed timestamp outcomes and the adapter tests for parsing and response translation; done means unchanged editor behavior, precise resulting Concert State, and updated documentation and Change Record.