projects: add The Mirror pull-through registry case study; wire cover art for both learn-site projects

Adds mirror-registry as a featured project (order 26, beside The Exploded
Cluster) covering the zot deployment, the bootstrap and auth circular
dependencies, and the retention/revalidation behaviours that corrected the
first published write-up. Adds the cover: field to exploded-cluster (it had
none, so the card rendered imageless) and stages public/covers/image.md with
the generation prompts and processing steps for both covers.

public/covers/image.md

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+# Project cover prompts - bztmon.com
+
+Two covers to generate: **The Exploded Cluster** and **The Mirror (pull-through registry)**.
+Paste the block below the `---` for each one into Gemini / Nano Banana 2, generate ~4
+variations, curate against the reject list, then process per "After generating".
+
+Aspect: **3:2 landscape** (existing covers are 1600 x 1074).
+
+---
+
+## House style block (already inside each prompt - do not paste separately)
+
+The site's covers are dark isometric hero renders: matte dark hardware, cyan and magenta
+edge light, a subtly circuit-etched ground plane, thin atmospheric haze, one small four-point
+sparkle highlight, and generous empty space on one side so the card's title text has room.
+Unlike the learn-site scene plates, covers MAY have a ground plane and atmosphere - they are
+never sliced or animated.
+
+## Reject checklist (per candidate, before you keep it)
+
+- [ ] any text, letters, numbers, logos or UI chrome anywhere in the frame (reject hard)
+- [ ] subject clipped by the frame edge, or centred so tightly there is no negative space
+- [ ] colours drifting warm - the palette is cyan/magenta on near-black, no orange or gold
+- [ ] photographic realism or stock-render gloss (it should read as an illustration)
+- [ ] busy background competing with the subject
+- [ ] the two covers looking like the same object (they must be distinguishable as a pair)
+
+## After generating
+
+1. Keep the best of each, name them `exploded-cluster.png` / `mirror-registry.png`.
+2. Convert and place (from the repo root, on the box with the site checkout):
+
+```
+# full-size cover: 1600px wide
+ffmpeg -y -i exploded-cluster.png -vf "scale=1600:-2" -q:v 80 public/covers/exploded-cluster.webp
+ffmpeg -y -i mirror-registry.png  -vf "scale=1600:-2" -q:v 80 public/covers/mirror-registry.webp
+
+# card thumbnail: 640px wide
+ffmpeg -y -i exploded-cluster.png -vf "scale=640:-2" -q:v 80 public/covers/thumb/exploded-cluster.webp
+ffmpeg -y -i mirror-registry.png  -vf "scale=640:-2" -q:v 80 public/covers/thumb/mirror-registry.webp
+```
+
+3. Both project files already reference these names (`cover: "exploded-cluster"`,
+   `cover: "mirror-registry"`), so the cards pick them up on the next build.
+
+---
+
+# PROMPT 1 - The Exploded Cluster
+
+Dark isometric technical illustration in a cinematic sci-fi engineering style, 3:2 landscape.
+Near-black background, hex #070b14, with a faint dark circuit-etched ground plane beneath the
+subject and a thin drift of atmospheric haze in the upper right. Matte dark metal and smoked
+glass surfaces with glowing neon edge lighting: primary cyan #3fbaf5, secondary magenta
+#e879f9, cool white rim highlights. Even studio lighting, no lens flare, no depth-of-field
+blur. ABSOLUTELY NO text, letters, numbers, logos, labels, arrows or UI elements anywhere in
+the image - surface markings must be abstract geometric patterns only.
+
+The subject: a single machine caught mid-explosion, its layers separated vertically like an
+engineering teardown diagram, floating apart with clear gaps of void between them. From the
+bottom up: a heavy dark chassis base with cyan-lit vents; above it a thick slab housing rows
+of small identical cells; above that two thin translucent glass sheets etched with fine
+circuit tracery, glowing faintly cyan; and at the top, held well clear of everything else, a
+small bright magenta-lit cube - the smallest and most luminous part, clearly the payload the
+whole machine exists to carry.
+
+The parts hover in stable formation rather than flying apart violently - a considered
+teardown, paused for inspection. Composition sits left of centre, leaving open dark space on
+the right third of the frame. One small four-point white sparkle highlight in the lower right,
+far from the subject.
+
+---
+
+# PROMPT 2 - The Mirror (pull-through registry)
+
+Dark isometric technical illustration in a cinematic sci-fi engineering style, 3:2 landscape.
+Near-black background, hex #070b14, with a faint dark circuit-etched ground plane beneath the
+subject and a thin drift of atmospheric haze in the upper left. Matte dark metal and smoked
+glass surfaces with glowing neon edge lighting: primary cyan #3fbaf5, secondary magenta
+#e879f9, cool white rim highlights. Even studio lighting, no lens flare, no depth-of-field
+blur. ABSOLUTELY NO text, letters, numbers, logos, labels, arrows or UI elements anywhere in
+the image - surface markings must be abstract geometric patterns only.
+
+The subject: a wide, low, heavily-armoured way-station machine sitting on the ground plane,
+right of centre - a bunker-grade cache with a glass-fronted shelf running along its face,
+holding a row of small identical luminous cyan bricks in cold storage. Its face has a single
+broad output aperture ringed in cool white light, aimed left.
+
+Above and behind it, arranged in a loose arc receding into the haze, five small dark intake
+pods at clearly diminishing scale - each a compact rounded capsule with one cyan-lit aperture,
+each floating alone with generous void between them, none touching the main machine. They
+read as five distant sources feeding one local shelf.
+
+Below and left of the way-station, close to the ground plane, a single luminous cyan brick
+hovers in mid-transfer - small, bright, clearly in motion toward the viewer's side of the
+frame, the one thing the whole arrangement is delivering.
+
+Composition sits right of centre, leaving open dark space on the left third for title text.
+One small four-point white sparkle highlight in the upper right, far from the subject.