fix: keep Memory label visible above the memory graph

[rustfuture]

Problem

demos/verification_2026-09-11.md records that the Memory: ... label is drawn but never visible in the rendered TUI. Root cause in src/ui.rs: the label was drawn at mem_rect.y (line 160) and the memory graph's clear pass then rewrote every cell in mem_rect.y..mem_rect.y + mem_rect.height (lines 182-185), followed by draw_braille_line, which also rewrites every cell in the rect. The network label survives because it is drawn at net_rect.y - 1, outside its graph rect.

New behavior

• The memory label is drawn one row above mem_rect, via a small testable helper DashboardLayout::memory_label_y() (src/rendering.rs). Layout rects, graph heights, y-scaling, and all other labels are unchanged.
• New pure regression test tests/tui_memory_label.rs: for multiple terminal sizes it asserts memory_label_y() is outside the memory graph rectangle (clear/braille region) and below the CPU graph clear pass. It fails on the pre-fix value (memory label row 10 overlaps memory graph rows 10..14 at 60x20).
• Network label semantics are untouched: Network I/O: RX .. KB TX .. KB (last interval) remains a per-interval byte delta, no KB/s.
• New capture pair under demos/ (tui_capture_2026-09-11-memory.raw / .txt) plus build identity in demos/tui_capture_2026-09-11-memory.md. Older captures and READMEs are untouched.

Test commands and evidence

cargo fmt --check                                      PASS
cargo check --locked --all-targets                     PASS
cargo clippy --locked --all-targets -- -D warnings     PASS
cargo test --locked                                    PASS (45 lib + 2 bin + 13 integration + 1 new, 0 failed)
cargo bench --locked --no-run                          PASS

Real render under a pty (binary built from 42f4602, clean tree), rendered with pyte 0.8.2 at 110x50:

python3 -m venv /tmp/grainx-c-render-venv
/tmp/grainx-c-render-venv/bin/pip install pyte==0.8.2
/tmp/grainx-c-render-venv/bin/python demos/capture_tui.py \
  target/debug/grainx \
  demos/tui_capture_2026-09-11-memory.raw \
  demos/tui_capture_2026-09-11-memory.txt \
  5 110 50

Final frame now contains (row 18): Memory: 52.4% (12.6GB/24.0GB), and row 26 keeps
Network I/O: RX 14.0 KB TX 1.0 KB (last interval). Binary SHA-256
8572c7e189a5736733869c337482a6a348838c9148d44831ff1048f991f7bbcc, rustc 1.94.1.

Limits

• Values are one host and one instant; the host snapshot contains real process names, PIDs, memory and disk labels (no credentials/paths/tokens; pattern scan clean).
• The CPU Usage: label has the same pre-existing overlap with the CPU graph clear pass; out of scope for this fix and noted in the capture doc.
• No change to data semantics or other UI sections; no README changes (owned by others).