feat: compress MicroCode's images with //% packable

[humanapp]

Moves MicroCode's images onto ui-core's new //% packable image compression (see related PR in microbit-apps/ui-core). 115 images now ship as one compressed pack instead of individual bmp literals: pixel data goes from 18,328 B to 9,922 B, and the V2 image from 475,236 B to 457,468 B (-17,768 B) against main.

//% packable
//% whenUsed
export const thermometer = bmp`...`

Images are still authored as literals in assets.ts. img.keys.json maps MicroCode's keys (tile ids and string ids) to them, npm run img:gen generates img.g.ts, and icons.get looks them up with _img.get(key).

What changed

• Icons. The 98 tile and editor icons are back in assets.ts as //% packable + //% whenUsed literals, restored from the earlier move to scripts/icon-literals.json.
• ui-core's icons. 12 of ui-core's icons that MicroCode uses are absorbed into its pack by qualified name ("ui.thermometer"), so they no longer ship as separate literals. ui.MISSING stays a literal, as the fallback.
• Lookup. icons.get resolves the six operator glyphs and MISSING itself, then everything else through _img.get.
• Direct references. Two images were referenced by const, which prevents packing: the editor's disk button now uses icons.get("disk"), and numberToImage's num2image array is replaced by _img.get("blocks" + n), so the classic-mode number images are decoded only if classic mode shows them.
• Name collision. A packed melodyEditor literal collided with icondb.melodyEditor, which is drawn from a melody at startup. The packed one is now melodyEditorIcon, and tid 179 points at it.
• Web-only samples. The sample-gallery images now have //% whenUsed. This annotation keeps them off the device.
• Unused images. 11 literals nothing uses on the device (btn_stop, settingsGear, loud, quiet, arith_plus, arith_equals, one to five) are now //% whenUsed, and drop out.
• Every image literal that isn't packed got a comment explaining why.

Heap

A packed image is decoded into the heap on first use and cached for the rest of the run. In MicroCode: if every image is decoded, that's 18,328 B. Measured on a microbit with every packed image decoded at boot, the GC heap still had 13,380 B free and the app ran normally. The classic-mode number images cost heap only if classic mode is used.

Testing

• Every packed icon decodes to pixels identical to main's icon table (98/98), checked against its uncompressed data.
• The post-build check finds no image shipping twice.
• On a micro:bit V2, every icon renders correctly on the home screen, the editor and the tile pickers.
• Classic mode can't be reached in the UI (HOME_ADD_SETTINGS is false), so the number images were checked with a diagnostic build that forced it on: all five matched checksums computed from their source literals.
• The committed language hexes are current: microcode-v2.en.hex matches a fresh build byte for byte.

Other changes

• README.md: the hex download link pointed at assets/microcode-v2.hex, a hand-committed build from November 2025. It now points at assets/microcode.hex, which the release workflow builds and commits.
• .github/copilot-instructions.md: same filename correction.
• Removes the stale assets/microcode-v2.hex.
• package.json: adds img:gen, and removes build:hardware:v2, which passed a --variant flag the current makecode CLI doesn't accept.
• Dependencies: ui-controls #v0.0.7 -> #v0.0.10 (pxt) and @microbit-apps/ui-core #v0.0.6 -> #v0.0.9 (npm).
• Regenerated language hexes in assets/hex/.

Merge order

Merge after ui-core v0.0.9 and ui-controls v0.0.10 are tagged; until then CI can't resolve the new refs. Then npm install to refresh package-lock.json and the img-gen bin link.