README: background, guarantees, value hashes, comparison with other approaches

[williamstein]

Rewrites the design parts of the README:

• Background: differential sync (~2013) → a linear sequence of patches ordered by one clock (edit history, but a late patch with an earlier time changed a version someone had already seen) → a DAG of patches like Mercurial → exact values, hashes and recorded merges (2026).
• Conceptual model and guarantees: every version is reproducible, divergence is detected, history never depends on the merge algorithm. Replaces the outdated "newest common ancestor + replay" description.
• Value hashes: a new section; the README did not mention them.
• How Patchflow compares: a table covering differential sync, OT, CRDTs, Mercurial/git and Patchflow, plus strengths and honest trade-offs (intent inferred from diffs, every replica merges, cost with many concurrent writers, rich-text serialization).
• Merges the two Status sections; sections reordered so the model comes before the implementation details.

Documentation only.