New techniques for storing state in querystring

docs/tree-state-serialization.md

@@ -24,6 +24,24 @@ base-128 variable-length integers.
 This adaptive layout was adopted before deployment, so it uses serialization
 version `1`; no dense-only version 1 URLs need backward compatibility.
 
+### Selectable field encodings
+
+Websites select their encoding with the CheckboxTree `stateEncoding` option.
+The value is intentionally not stored in the fragment, so it forms part of the
+website's URL-format contract. Version 1 supports:
+
+* `adaptive` (default): adaptive `c` and `x` fields described above
+* `dense`: trimmed dense `c` and `x` bitsets
+* `tiered`: hierarchical checked fields and depth-grouped expanded fields
+
+In tiered encoding, `c1` stores actual root checked state. Each deeper `cN`
+stores whether a node differs from its direct parent's checked state;
+restoration uses XOR. Expansion does not inherit from a parent: each `xN` stores
+actual expanded state for branches at depth N, with collapsed as the baseline.
+Every tier independently uses the adaptive representation. Non-selectable
+parents provide a false checked baseline. An `n` field records the manifest slot
+coverage at serialization time; slots appended beyond that value restore false.
+
 ## 1. Purpose
 
 Implement a compact, shareable, client-side serialization format for storing tree widget state in a URL.