fix(sync): survive push heap exhaustion and instrument the push path

Run 4 (2026-07-13): remote.push() consumed ~6 MB of PSRAM over 66 s and
the UI thread aborted on Frame::new_white's per-draw vec![0xFF; 26928].
Run 5 confirmed the UI now survives, but the push still exhausts both
pools (a ~7 KB inflateInit fails inside the pack build) and the heap
telemetry never fired — hence the telemetry rework.

- display/editor: Editor::draw_into() renders into a caller-owned
  Frame; main.rs keeps two boot-time frames (shown/back) and mem::swaps
  on success, so steady-state repaints never allocate.
- git_sync: odb cache capped at 1 MB via raw libgit2-sys opts (git2
  0.20 doesn't wrap the total cap); log_push_heap at pre-push,
  post-push and push-failure with largest-PSRAM-block and per-pool
  min-evers; pack_progress logging is now time-gated (2 s) — the
  count gate (+256 objects) never fired because AddingObjects reports
  total=0 and a small push inserts only dozens of objects.
- editor: empty/whitespace snippets file parses as 0 snippets instead
  of a JSON error at boot.
- build.rs: refuse a git-feature build with unset TW_* publish vars —
  a bare `cargo build --features git` baked empty creds and produced a
  firmware whose :sync could never work.

editor/src/lib.rs

@@ -352,9 +352,13 @@ impl Snippets {
     /// display name, each value `{ prefix, body, description? }` where `body` is a
     /// string or an array of lines. Labels in `${n:label}` stops are stripped to
     /// `$n`. Entries come back sorted by name (a `BTreeMap` parse — deterministic,
-    /// and the empty-`$` palette order; a query re-ranks by fuzzy score anyway). A
-    /// malformed file is an `Err` the host logs before booting with no snippets.
+    /// and the empty-`$` palette order; a query re-ranks by fuzzy score anyway). An
+    /// empty (or whitespace-only) file means "no snippets", same as a missing one —
+    /// only a malformed file is an `Err` the host logs before booting with none.
     pub fn parse(src: &str) -> Result<Self, serde_json::Error> {
+        if src.trim().is_empty() {
+            return Ok(Self::default());
+        }
         let raw: std::collections::BTreeMap<String, RawSnippet> = serde_json::from_str(src)?;
         let snippets = raw
             .into_iter()
@@ -3166,11 +3170,25 @@ impl Editor {
     /// text (e.g. a boot message). View never draws a caret. In the main loop
     /// Normal always passes `true`, so its block caret is unaffected.
     pub fn draw(&mut self, cursor_on: bool) -> Frame {
+        let mut f = Frame::empty();
+        self.draw_into(&mut f, cursor_on);
+        f
+    }
+
+    /// [`draw`](Self::draw) into a caller-owned frame, reusing its allocation.
+    /// Firmware keeps two boot-time frames and round-trips them through here so
+    /// a repaint never allocates: the editor must stay drawable even when a
+    /// background `:sync` push has taken the heap to the floor — a failed
+    /// framebuffer alloc aborts the whole app (the 2026-07-13 OOM).
+    pub fn draw_into(&mut self, out: &mut Frame, cursor_on: bool) {
         let lay = self.layout();
         let (crow, ccol) = self.caret_rc(&lay);
         self.adjust_scroll(crow, lay.len());
 
-        let mut f = Frame::new_white();
+        // Take the caller's buffer (no copy, no alloc), render into it as an
+        // owned frame — the body predates this signature — and hand it back.
+        let mut f = std::mem::replace(out, Frame::empty());
+        f.clear_white();
         let text_style = MonoTextStyle::new(&FONT_10X20, BinaryColor::On);
         let gutter = self.gutter_cols();
         let cols = WRITE_COLS - gutter; // text columns after the gutter
@@ -3335,7 +3353,7 @@ impl Editor {
         if self.prefs.theme == "dark" {
             f.invert();
         }
-        f
+        *out = f;
     }
 
     /// Draw the side panel: a full-height rule, word count at the top, and the
@@ -5910,6 +5928,8 @@ mod tests {
     #[test]
     fn parse_snippets_empty_and_malformed() {
         assert!(Snippets::parse("{}").unwrap().0.is_empty());
+        assert!(Snippets::parse("").unwrap().0.is_empty()); // empty file = no snippets
+        assert!(Snippets::parse(" \n\t").unwrap().0.is_empty()); // whitespace-only too
         assert!(Snippets::parse("{ not json").is_err()); // host logs, boots with none
     }