docs(known-issues): make it the single source of truth

Promote known-issues.md to the authoritative divergence reference ahead
of retiring development/vimwiki-gap.md:
- drop the pointer into vimwiki-gap.md so the file stands alone
- reframe the intro as the single source of truth
- add an "Internal deferrals (non-parity)" section capturing the
  cold-path optimizations (RSS render_entry_body disk read,
  push_level_edit_for_line offset scan, diary neighbor scan) that
  otherwise only lived in the gap doc, so nothing is lost on deletion

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

known-issues.md

@@ -5,10 +5,9 @@ not a fork of the original VimL plugin. It targets behavioral parity with
 vimwiki, but a handful of upstream options and behaviors are implemented
 differently — or intentionally not at all — because of that architecture.
 
-This page lists what to expect if you're coming from vimwiki. Everything here
+This page is the single source of truth for how nuwiki differs from vimwiki.
-is either a deliberate design choice or a tracked future task; none of it is an
+Everything here is either a deliberate design choice or a tracked future task;
-accidental bug. For the detailed, per-option parity log (with code references
+none of it is an accidental bug.
-and test names), see [`development/vimwiki-gap.md`](development/vimwiki-gap.md).
 
 If you hit a behavior that *isn't* listed here and doesn't match vimwiki, please
 file it — that's a real bug.
@@ -108,6 +107,25 @@ self-contained. You can set any of these explicitly to match upstream exactly.
 
 ---
 
+## Internal deferrals (non-parity)
+
+These are not vimwiki differences — they're known low-priority optimizations on
+cold code paths, recorded so they aren't rediscovered as "bugs." All are
+correct as-is; the deferral is a deliberate risk/reward call.
+
+- **RSS `render_entry_body` reads from disk.** It re-reads and re-parses each
+  diary entry from disk rather than reusing an open buffer. This is the *correct*
+  source of truth for an export artifact — switching to unsaved buffer text would
+  change semantics, not just performance.
+- **`push_level_edit_for_line` scans from offset 0.** The list-level change
+  (`gl`/`gL`) computes each line's start offset by scanning from the document
+  start. It's an interactive single-keypress path over small inputs, and the
+  offset-arithmetic rewrite carries more regression risk than the saved time is
+  worth.
+- **Diary next/prev neighbor scan.** `:VimwikiDiaryNextDay` / `PrevDay` rebuild
+  and sort the entry list to find one neighbor. Same reasoning — cold path,
+  small input.
+
 ## Notes
 
 - **Third-party plugin compatibility.** A shim at `autoload/vimwiki/vars.vim`