fix: correct path navigation, map-nested writes, and CRDT Text convergence

[brunoga]

internal/core/path.go:

• Navigate: check keyed-slice unconditionally before falling back to positional index, fixing non-numeric keys (e.g. "todo", "in-progress") that were never resolved because the old guard required IsIndex=true
• Set/Delete: replace ResolveParent+switch with recursive setAtPath/ deleteAtPath that copy-modify-put-back at every map boundary, fixing silent write loss when a value is nested inside a map
• makeMapKey: extracted helper with Convert for named-string and uint key types, fixing a panic on uint-keyed maps

crdt/crdt.go:

• Replace post-apply full-tree remergeTextFields walk with per-op textAncestorPath detection during the Merge filter loop; Text field paths are collected in O(depth) per op and merged via MergeTextRuns after the LWW apply — no tree walk, no index-alignment bug on keyed slices
• Remove the early break in textAncestorPath so that TextRun sub-field ops (e.g. /Body//Deleted, depth 3) correctly walk up to the Text ancestor

Tests added for each fix in path_test.go and crdt_test.go.