mecatui: small startup-memory wins (mascot decode, retained transients) #2412

Problem

Beyond the embedded daemon (#2408), mecatui's startup RSS (~110 MiB in connect mode) includes avoidable memory:

• Mascot: welcome/embed.go keeps the decoded mascot image for the whole process lifetime (mascotOnce): ~6 MiB of heap, used only by the splash. The half-block path also decodes the full-resolution image (another ~6 MiB transient plus file pages).
• Retained startup transients: startup allocates 52–58 MB of short-lived data (13 MB in a minimal client), and the Go runtime keeps ~18 MiB of it as idle heap.

Proposal

1. Don't memoise the decoded mascot; drop it once the splash is gone (−6 MiB heap).
2. Embed a ≤ 256 px copy for the half-block path and keep full resolution only for kitty graphics (welcome goldens need updating).
3. Return startup transients to the OS: call debug.FreeOSMemory() once after the first frame (estimated −10 to −20 MiB at startup), or set a lower GOGC (GOGC=50 measured −7 MiB at startup, with ~2× the GCs).

Acceptance

• Startup RSS in connect mode drops by ≥ 15 MiB.
• The splash looks the same (goldens).