handoff: Phase 3 complete (all production .cs <= 500), push pending for A-E + docs

Rewrite HANDOFF.md: Phase 2 is pushed at 1f4624c (stale claim corrected);
Phase 3 commit rows A-E + Controls/index.md listed as local-only, push pending
on explicit approval. Records the re-structuring principle (split by concern,
500-line count is a ceiling not the goal) and the session's truncation-accident
lesson. MyMistakes.md gains the never-awk-into-the-source-you-read rule.

MyMistakes.md

@@ -44,3 +44,26 @@ packages, high-quality downscale via `TransformedBitmap`. Screenshots compress
 4. Point `README.md` at the `.jpg` (not `.png`).
 
 **Result:** 3.2MB screenshot → 1400×794 → **188KB** `ytLlive-preview.jpg` in `docs/`.
+
+---
+
+## Splitting a large file into partials — NEVER `awk … > SRC` while awking SRC
+
+(2026-08-31, Commit D) Tried to split `SocialsDialogViewModel.cs` in one line:
+`{ awk '…' SRC; echo ""; awk '…2…' SRC; } > SRC`. The **first write truncated
+SRC to 1 line**, so the second `awk` read the already-truncated file → the whole
+source was lost (1 line left). Recovered with `git checkout -- SRC`, then redid
+it, but the same bug could have meant making it up from scratch.
+
+**Rule:** when a cut needs N blocks from one source into N files, never write a
+block back onto the source that the `awk`s still read. Instead:
+1. Read the source **once** at the start into temp files (`mktemp -d`, one file
+   per block), with a `$D` variable you carry forward.
+2. Verify block sizes (`wc -l`) and brace balance (`python3 -c` counting `{`/`}`)
+   before touching any real file.
+3. Then assemble each new file from `cat D/block …` — never truncating the source
+   until every read is done.
+
+`git status` can't save you here if you don't notice until the file is gone —
+`git checkout -- <path>` from the last commit is the recovery. Cheap insurance:
+restore-then-retry, do it atomically from temp files the first time.