Gemini: read big pages from the card as you scroll (Q88)

A page larger than memory allows is now windowed instead of cut: one
pass over its file (cache or Saved Page) counts lines, indexes every
64th (offset, and the preformatted state there in bit 31: about 1 KB
for a 1 MB page) and loads the first window. Scrolling near either end
reads the next or previous window on the Gemini task; the line on top
of the screen stays put, the scrollbar follows the whole page, and Tab
at a window's edge pages on instead of wrapping. Window budgets count
the memory the old window gives back.

Display pages alternate between two cache files so the one on screen
is never overwritten by the next fetch; jobs use a third.

On the device, Cosmos with IRC connected: 226 of 419 lines at first,
then lines 192-419, back to 64 and 0 while scrolling. `key space` added
to the console's key command.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

docs/milestones/G1.md

@@ -27,6 +27,7 @@ Gemini (geminiprotocol.net): one request per TLS connection on port 1965, the re
 | Q84 | The start page lists Saved Pages, newest first, grouped by capsule; they open with no network. In a Saved Page, a link to another Saved Page opens the saved copy; other links fetch online if Wi-Fi is up, or say "not saved, offline". A Saved Page shows when it was saved; `r` refreshes it. |
 | Q86 | *Decided after step 1.* **Two floors:** free heap stays above 40 KB in steady state, and above 20 KB for the second or two of a TLS handshake (measured: 24 KB with IRC connected). A fetch refuses to start below **55 KB** free ("not enough memory: stop IRC or retry"), so nothing pushes lower. |
 | Q87 | *Decided in step 3.* **With a card, every page streams to `/gemini/cache/page.gmi`** in 1 KB pieces while its TLS connection is open; once the connection closes and its ~45 KB is back, the page is loaded into RAM as far as the 40 KB floor allows. The whole page stays on the card (Saved Pages copy it). Without a card, the page goes straight to RAM under the same two floors. Pages are held as lines in 4 KB chunks, never one large block (the largest free block with IRC connected is about 31 KB). |
+| Q88 | *Added after step 6.* **A page bigger than memory allows is read from the card as you scroll.** Opening it, one pass over its file counts the lines, records where every 64th starts (and whether it's inside a preformatted block), and loads the first window. Scrolling near either end of the window reads the next or previous one in the background, keeping the line on top of the screen where it is; the scrollbar follows the whole page. Display pages alternate between two cache files, so the one on screen is never overwritten by the next fetch; background jobs use a third. |
 | Q85 | Saved Pages are deleted from the App only (`d`, with confirmation), never by Storage Clean-up's age rules, like Notes. |
 
 ## Measured (step 1)
@@ -36,6 +37,8 @@ Gemini (geminiprotocol.net): one request per TLS connection on port 1965, the re
 - **Step 3, Cosmos (31.6 KB) with IRC connected:** first stopped at 4.6 KB (RAM only, the transfer's 20 KB floor). Streamed to the card: the whole page on the card, 20 KB of it loaded, lowest free heap 19.5 KB during the transfer and 43 KB once loaded. Without IRC: the whole page in RAM. Redirects (Cosmos `31`), input (`10`), not found (`51`) and a changed certificate (refused, both fingerprints shown) all checked on the device.
 - **Steps 4–6 on the device:** Project Gemini and its relative links, Back with the scroll restored, a refused YouTube link; `b` bookmarks, `s` saves (and says when it replaced an older copy), `S` saved 6 of 6 pages, the start page lists both; a Saved Page opens from the card with its origin, its saved links open saved copies, `r` refreshes, `d` asks first; Kennedy's input prompt sent "cardputer" and got 87 results; emoji drawn as `?`.
 - **A bug found there:** refreshing first loaded the whole Saved Page into RAM just to read its origin, next to the App's copy and a TLS connection: the heap fell to 436 bytes. Now only the first line is read, and every fetch (pages, saves, refreshes) checks the 55 KB start floor. Lowest since boot afterwards: 53.8 KB.
+- **Windowed pages (Q88), Cosmos with IRC connected:** 226 of 419 lines in memory at first; paging down loaded lines 192–419 in one window, scrolling back up loaded 64 onwards, then 0 onwards. Window budgets count the memory the old window gives back.
+- **Open:** with IRC connected, the lowest free heap during a Gemini fetch was 13–15.5 KB in later runs (24 and 19.5 KB earlier): the TLS receive buffers vary with the server's records. That's under Q86's 20 KB transient floor; the 55 KB start floor doesn't guarantee it.
 - Antenna (`warmedal.se`) doesn't answer, from the PC either; the default aggregator becomes Cosmos (`gemini://skyjake.fi/~Cosmos/`, which redirects to `cosmos.skyjake.fi`).
 
 ## Done when