fix(core): a typed code without an alpha keeps the alpha in force (#116)

[Maxaubert]

Closes #116.

Owner, 2026-10-03: "yes", to keeping the alpha when a 6-digit code is typed. Before this, typing #203040 into a see-through Primary in Prism turned the window solid, because a typed code counted as a whole new colour, alpha included.

• The change: in colourCommit (core), a code that names no alpha (#rgb, #rrggbb, rgb(), hsl()) changes only the colour and keeps the alpha in force. A code that says an alpha (#rgba, #rrggbbaa, a fourth argument to rgba() or hsla()) sets it, solid included. The alpha bar is unchanged. This applies to every colour field in both apps.
• New pure helper: saysAlpha, with tests.
• Versions: core 0.22.1, app 0.27.1. Prism gets it through the automatic core update, whose terminal gate runs termColourPicker.

Verified (the PR gate):

• Typecheck and lint pass (0 errors). 1221 unit tests pass, including 3 new colourCommit / saysAlpha cases.
• E2E colourPicker (48 checks) and opacityAlpha (21) pass.
• Prism's own tests type a 6-digit code once, into the theme editor's Background. That field has no alpha in Prism, so its result doesn't change.

🤖 Generated with Claude Code

https://claude.ai/code/session_01FHHaWKR4M5QtW7Wecyuk4t