Docs: specify ownership-first reactivity model

[SavioCodes]

Summary

• add an ownership-first reactivity model spec for components
• define the recommended separation between owned state, bound inputs, and upward events/commands
• document how style and animation should fit without becoming hidden mutation channels
• align the existing reactive graph and IR docs with that ownership model
• wire the new spec into the repository documentation map

Why This Matters

Issue #114 is really about semantic direction, not just syntax.

Without a clear ownership model, later work on graph generation, IR lowering, style, and animation risks baking in assumptions that are hard to unwind.
This PR closes that gap by recording a predictable first implementation target:

• one owner per mutable value
• parent-to-child data flow for reactive inputs
• child-to-parent changes via explicit events/commands
• style as derived data
• animation as an effect/transition concern above the pure dependency graph

Scope

This PR is docs/spec only.
It does not claim that reactivity is fully implemented on main.
It intentionally keeps syntax details open where the repository has not committed to one final surface yet.

Validation

git diff --check
cargo test --no-run
cargo clippy --all-targets --all-features -- -D warnings

Closes #114.