feat: reactivityMode

[runspired]

Sketches out reactivityMode and lays groundwork in types etc.

Should be relatively easy/quick to implement, and could be further optimized via batching/reduction of work generated by the graph and cache (some crossover with #9667).

Unanswered but not impossibly hard questions to resolve:

• do locals still get their own signal (likely yes)
• values that use their own signal as a value store ... maybe just document which and note these will still have one?
• can you create a record in immutable mode? (likely no, or it must be created "fully formed" and/or becomes immutable once no longer "isNew")