SPECS: describe one image id per poster footprint

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

SPECS.md

@@ -64,7 +64,7 @@ Notes:
   - Global speed / pause state in the status bar. Pause/resume and delete a SABnzbd job.
 - **Status bar / help:** the bottom bar lists the keys that work in the current view or panel; the top right shows the connection state of each service; `F9` opens the full key reference.
 - **Posters:** the detail panes show the poster at the top right; the head (title, facts, path) and the plot are one text that flows along the poster's left side and continues at full width below it (text is wrapped by arr-tui itself, with a narrower width for the first lines); on panes too small for the poster, the text simply uses the full width. Two ways to draw the poster, chosen by `ui.cover_style`:
-  - **Real images** (`kitty`, and `auto` in kitty/Ghostty): the Kitty graphics protocol with Unicode placeholders. The poster (kept up to 640 px tall) is uploaded once as PNG (`a=t,f=100,q=2`, 4096-byte base64 chunks), placed virtually with the cell footprint (`a=p,U=1,c=,r=`; a new size deletes and recreates the placement), and shown by writing U+10EEEE placeholder characters whose combining marks give row and column (the 297-entry table from the protocol) and whose foreground color is the image id. Since it is ordinary text, popups, dialogs and view changes cover it correctly and tcell's diffing keeps it in place. The footprint uses the real cell size in pixels (the window's pixel size divided by its cell count, rounded down to whole pixels so padding around the grid does not skew it; all images are uploaded afresh whenever the window or cell size changes) and a box whose shape is within 3% of the poster's; the remaining sliver is cropped from the poster's edges before upload (one upload per size, the previous one is deleted), because the terminal letterboxes an image that does not fit its box exactly. As a second safeguard the placeholder cells use the panes' background color, so anything the picture does not cover blends in (without it the terminal's own, slightly different, default background showed as grey bars beside the poster). Writes to the terminal go through the screen's tty on the UI goroutine, so they cannot interleave with drawing. Requires 24-bit color; otherwise, and inside tmux/screen, the block fallback is used. Terminals are recognised from `TERM`/`TERM_PROGRAM`/`KITTY_WINDOW_ID`/`GHOSTTY_RESOURCES_DIR`. The go-termimg library was looked at and not used: its placeholder support is marked experimental, its diacritic table does not match the protocol's, and it writes images straight to the terminal instead of through the screen.
+  - **Real images** (`kitty`, and `auto` in kitty/Ghostty): the Kitty graphics protocol with Unicode placeholders. The poster (kept up to 640 px tall) is uploaded as PNG (`a=t,f=100,q=2`, 4096-byte base64 chunks) and placed virtually with the cell footprint (`a=p,U=1,c=,r=`) right away. Every footprint (crop and cell size) is its own image id, placed once and never changed: Ghostty kept showing the old, small placement when the size of an existing one was changed, so a new size uploads under a new id and deletes the old one. The poster is shown by writing U+10EEEE placeholder characters whose combining marks give row and column (the 297-entry table from the protocol) and whose foreground color is the image id. Since it is ordinary text, popups, dialogs and view changes cover it correctly and tcell's diffing keeps it in place. The footprint uses the real cell size in pixels (the window's pixel size divided by its cell count, rounded down to whole pixels so padding around the grid does not skew it; all images are uploaded afresh whenever the window or cell size changes) and a box whose shape is within 3% of the poster's; the remaining sliver is cropped from the poster's edges before upload (one upload per size, the previous one is deleted), because the terminal letterboxes an image that does not fit its box exactly. As a second safeguard the placeholder cells use the panes' background color, so anything the picture does not cover blends in (without it the terminal's own, slightly different, default background showed as grey bars beside the poster). Writes to the terminal go through the screen's tty on the UI goroutine, so they cannot interleave with drawing. Requires 24-bit color; otherwise, and inside tmux/screen, the block fallback is used. Terminals are recognised from `TERM`/`TERM_PROGRAM`/`KITTY_WINDOW_ID`/`GHOSTTY_RESOURCES_DIR`. The go-termimg library was looked at and not used: its placeholder support is marked experimental, its diacritic table does not match the protocol's, and it writes images straight to the terminal instead of through the screen.
   - **Block characters** (`quadrant`, `sextant`, and the fallback): every cell shows several pixels in the best two-color split (`quadrant`: 2×2 pixels, glyphs from the Block Elements range every font has; `sextant`: 2×3 pixels, sharper, glyphs from Symbols for Legacy Computing). `auto` picks sextants on kitty, WezTerm, foot and Ghostty, and on other terminals when fontconfig lists a font with the sextant characters (`fc-list :charset=1fb00`, about 20 ms at startup; terminals draw glyphs their font lacks from fallback fonts; skipped over SSH because the installed fonts are the remote machine's), and quadrants otherwise.
   Posters are JPEG, PNG, GIF or WebP (Radarr and Sonarr keep whatever bytes the provider sent under a `.jpg` name, which can be WebP), decoded and downscaled in the process; for any other format the Log says what the data looked like (e.g. AVIF, an HTML error page, or its first bytes). Library items fetch `/MediaCover/<id>/poster-250.jpg` from their Radarr/Sonarr instance with the API key (then the original `poster.jpg`; an instance that answers 200 with an HTML page, i.e. wants a login for `/MediaCover`, is also tried with the key as the `apikey` query parameter; and if the instance has no usable copy, the provider's `remoteUrl`, fetched without credentials); search results fall back to the provider's `remoteUrl` (fetched without credentials). Posters are cached in memory (the terminal's copies are deleted when the cache is dropped and on exit), failed downloads are not retried for two minutes and are written to the Log (F7) with the reason, and the poster shrinks or disappears when the pane is too short. `ui.covers: false` disables it.
 - **Calendar view (F5):** a two-week window (starting today, moved a week at a time with `[`/`]`, `t` returns to today) with the library's movie releases and airing episodes, grouped by day. Radarr `GET /calendar` supplies the cinema, digital and physical release dates (taken as written: midnight UTC means that calendar day) and Sonarr `GET /calendar?includeSeries=true` the episodes with their air time (shown in local time). Unmonitored items are included with `u`. A movie appears once per release date in the window. Each entry shows type, title, episode or release kind and status (downloaded / missing / upcoming); the detail pane shows poster and plot; `Enter`/`i` starts the release search and `s` an automatic search.