Implement OSC 52 clipboard forwarding

[pcarrier]

Summary

• capture alacritty OSC 52 ClipboardStore events in the terminal driver
• expose and drain pending clipboard stores in the server tick loop
• forward OSC 52 text through the existing S2C_CLIPBOARD_CONTENT browser clipboard path
• add a focused OSC 52 unit test

Tests

• direnv exec . cargo test -q -p blit-alacritty
• direnv exec . cargo check -q -p blit-server

[github-actions]

Coverage

Crate	Lines	Functions	Regions
alacritty-driver	60.4% (545/902)	64.2% (43/67)	60.7% (782/1289)
browser	0.0% (0/807)	0.0% (0/65)	0.0% (0/1370)
cli	28.1% (1208/4299)	41.3% (171/414)	31.1% (2182/7019)
compositor	1.0% (93/9234)	2.0% (8/400)	1.2% (146/12408)
fonts	76.8% (486/633)	85.5% (47/55)	77.9% (922/1183)
gateway	25.7% (362/1411)	29.0% (36/124)	19.4% (449/2318)
proxy	18.3% (150/818)	20.9% (24/115)	20.4% (260/1277)
remote	71.7% (1975/2756)	81.7% (188/230)	74.3% (3737/5032)
sd-notify	72.8% (67/92)	100.0% (6/6)	82.4% (108/131)
server	18.7% (2184/11673)	33.6% (251/746)	20.2% (3656/18131)
ssh	1.9% (7/374)	3.2% (1/31)	0.7% (4/613)
webrtc-forwarder	2.7% (72/2624)	2.1% (4/187)	1.2% (50/4335)
webserver	63.5% (753/1185)	70.8% (121/171)	67.8% (1380/2034)
Total	21.5% (7902/36808)	34.5% (900/2611)	23.9% (13676/57140)

[github-actions]

🔗 Preview: https://blit-qiwihxro7-indent.vercel.app