Track terminal used rows

[pcarrier]

Adds store-only terminal used-height tracking on top of #63.

• Tracks ever-used visible rows in the alacritty driver.
• Resets on terminal reset / clear-like sequences and caps on resize.
• Broadcasts S2C_USED_ROWS to clients and stores usedRows on core sessions.
• Does not change layout behavior yet.

Validation:

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

[github-actions]

🔗 Preview: https://blit-arulkhm6z-indent.vercel.app

[github-actions]

Coverage

Crate	Lines	Functions	Regions
alacritty-driver	63.7% (626/982)	67.1% (49/73)	64.3% (919/1429)
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	71.5% (1975/2763)	81.4% (188/231)	74.1% (3737/5045)
sd-notify	73.9% (68/92)	100.0% (6/6)	83.2% (109/131)
server	18.7% (2186/11701)	33.6% (251/748)	20.1% (3660/18180)
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	21.6% (7986/36923)	34.6% (906/2620)	24.1% (13818/57342)