docs: editor coverage — version-2 B-trees, shrinking, dense attributes, reuse

CHANGELOG (Unreleased): the new FileEditor operations, space reuse, and
the two reader fixes (implicit index grid, object-header continuation
chains). known-issues: the editor's remaining refusals (skipped heap
blocks, heaps with filters or child indirect blocks, freeing a heap
block, implicit-index insertions, ...) and the append-waste sizes before
and after reuse (measure_append_waste, tank 2026-09-26; file sizes are
deterministic). range-reads design: status note on the reader changes.
README and CLAUDE.md: what the editor covers and how to test it.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

docs/design/range-reads.md

@@ -3,7 +3,13 @@
 Status: proposal, 2026-09-26; the plan for Phase 3's largest architectural
 change. Progress: M1, first part (the `Storage` trait and the metadata
 parsers listed in `CHANGELOG.md` under "Range reads, milestone M1") is done;
-group B-tree v2 lookups, dense groups and the facade are not converted yet. Every count below was
+group B-tree v2 lookups, dense groups and the facade are not converted yet.
+Later the same day (branch `feat/p3-editor-coverage`) two reader fixes touched
+converted code without changing the plan: object-header continuation chunks
+are followed without recursion (still one bounded `read_at` per chunk), and
+implicit chunk indexes are addressed over the maximum chunk grid (in
+`chunked_read`, an M2 module). The in-place editor (`FileEditor`) keeps
+working on the whole file in memory; it is not part of this design. Every count below was
 taken on `tank` on 2026-09-26 at commit `de2a53f`, with the commands given
 next to it. No timing numbers appear here on purpose: the machine was shared
 with other build jobs when this was written.