The rule builder's Group nesting was capped at one level (#176's Kodi
model). Generalize it to recursive nesting bounded by a single shared
constant, MAX_GROUP_DEPTH (types.ts, = 5, root group is depth 0):
- parse.ts: replace the allowNested boolean with a depth counter that
recurses to the cap; deeper input stays out of subset (null -> raw-text
fallback), so parse remains the exact inverse of compile. Sub-group
detection now requires the leading '(' to be the one closed by the
trailing ')' (quote/escape aware), so '(a)x(b)' can't be mistaken for
one wrapped group.
- RuleBuilder.tsx: 'Add group' is offered while depth < MAX_GROUP_DEPTH
instead of only at the root; nested group boxes get box-sizing:
border-box so per-level padding can't overflow (no global reset).
- roundtrip.test.ts: the 500-tree generator nests to the cap and asserts
the corpus actually reached it; explicit depth-3 cases added to
compile/parse/validation tests and a depth-gate test to RuleBuilder.
compile.ts and validation.ts already recursed correctly and are unchanged.
No backend/OpenAPI change.
Docs: spa-conventions.md §12; decisions lifecycle — new active record
spa.rulebuilder-nesting, predecessor spa.smartcollection-rule-builder
relocated to docs/decisions/archive/spa.md as superseded.
fixes#436
