framing: GPU status band — local freshness + v8 StatusFacts

Q#S1 splits facts by authority: L:C and scroll derive locally (the
optimistic caret must not lag a round trip); name, modified, and
exact diag counts arrive via a new additive StatusFacts variant
(protocol v8). The reserved ModeLine(Vec<Cell>) stays grid-shaped
and unused. Q#S2 quad + second TextArea; Q#S3 one text_area_bottom
helper for the five bottom-assuming geometry sites. Three bets.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/pmacs-gpu-status-band-framing.md

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+# pmacs-gpu status band — framing pass
+
+Date: 2026-06-12. The GPU window has no status surface: no buffer
+name, no modified flag, no cursor position, no diagnostic counts
+(the M4.6 parity session deferred counts here for exactly this
+reason). Survey facts: `InstanceMessage::ModeLine(Vec<Cell>)` has
+sat reserved-but-unused since day one; `BufferSnapshot` carries no
+name/modified; glyphon's `prepare()` takes `&[TextArea]` so a second
+band area is structurally cheap; five geometry sites assume text
+runs to the surface bottom (`estimated_visible_lines`,
+`TextBounds.bottom`, the minimap height pair, the edge-scroll
+bottom band).
+
+## Q#S1 — where status facts come from
+
+**Stance: split by authority and freshness.**
+
+- **Locally derived, per frame**: cursor L:C (from `own_cursor` ×
+  `current_line_starts` — the *optimistic* caret, so the readout
+  tracks typing bursts instead of lagging a round trip behind
+  them), and the scroll indicator (`scroll_top` × visible ×
+  total, the TUI's All/Top/Bot/NN% formula).
+- **Instance-authoritative, on the wire**: buffer name, modified
+  flag, exact whole-file diagnostic counts. A new additive variant
+  (protocol v8, ladder continues on the v6 floor):
+  `StatusFacts { buffer_id, name, modified, diag_errors,
+  diag_warnings }`, emitted by the semantic producer when any fact
+  changes (cached-compare, like `FileStyleSummary`; counts piggyback
+  the diag-store epoch from the parity session).
+
+Rejected: populating the reserved `ModeLine(Vec<Cell>)`. It is
+grid-shaped — pre-formatted styled cells — which bakes the TUI's
+layout into a frontend that does its own, and a daemon-formatted
+L:C would visibly lag the optimistic caret. The variant stays
+reserved for grid use. Also rejected: counting marked lines in
+`FileStyleSummary` as the counts — that counts *lines*, the TUI
+counts *diagnostics*; two frontends showing different numbers for
+the same buffer reads as a bug.
+
+## Q#S2 — band rendering
+
+**Stance: quad + second `TextArea`.** A `STATUS_BAND_HEIGHT` (26px)
+strip at the surface bottom: a background quad through the existing
+quad pipeline, and a one-line cosmic-text `Buffer` shaped only when
+the composed status string changes, passed as a second `TextArea`
+in the same `prepare()` call. Left: `name *` (modified star).
+Right: `E:n W:n  L:C  NN%` — diag counts colored by the severity
+palette, omitted when zero.
+
+## Q#S3 — geometry
+
+**Stance: one helper, no scattered arithmetic.** `text_area_bottom
+(surface_height) = height - STATUS_BAND_HEIGHT` feeds every site
+that assumed the surface bottom: `estimated_visible_lines`,
+`TextBounds.bottom`, the minimap height pair
+(`minimap_band_contains` / `minimap_y_to_line` / painters), and the
+edge-scroll bottom band. The minimap keeps its own `MINIMAP_BOTTOM`
+inset *above* the band.
+
+## Predicted findings (categorical bets)
+
+1. **A missed bottom-assuming site**: something still measures to
+   the raw surface bottom — surfaces as content drawn under the
+   band, or a hit-test/scroll off-by-a-line at the window bottom.
+2. **Visible-lines ripple**: shrinking `estimated_visible_lines`
+   perturbs viewport/scroll math tuned to the old height — surfaces
+   as a flickering scroll indicator or an unreachable last line.
+3. **Optimistic L:C flicker**: the locally derived readout jumps
+   when a daemon `CursorByte` confirms through the floor-release
+   path mid-burst — cosmetic, but noticeable enough to get reported.
+
+## Session plan
+
+Order: geometry + band rendering with the local facts (L:C, scroll)
+→ v8 `StatusFacts` wire + daemon emission + name/modified/counts →
+tests/polish. Manual validation: resize the window, type a burst
+mid-file (L:C tracks), scroll to both extremes (Top/Bot), introduce
+and fix an error (counts appear/vanish), edit (modified star), and
+re-run the minimap + edge auto-scroll gestures against the new
+bottom edge.