Keep at least three screens of kitty images in the registry

[tomlm]

Problem

MaxImageRegistryBytes defaults to 32 MB and the registry trims the oldest images by id regardless of screen size. A client that tiles a full-screen picture (Consolonia's kitty renderer keeps up to three screens' worth of tiles in the terminal to show them again without resending) exceeds that from about 2560x1440 up, and a 4K screen alone is 33 MB. Once the registry has dropped a tile the client still counts on, placing it again silently shows nothing.

Change

The option becomes a floor: the kitty registry budget is max(MaxImageRegistryBytes, 3 × Cols × CellWidthPixels × Rows × CellHeightPixels × 4). Zero or less still means no limit, and a configured budget larger than three screens still applies. Only the two kitty store/charge sites use the registry budget for storage; the iTerm2 OSC sites keep using the option as a per-image sanity ceiling, so their tests at 1 and 1024 bytes are unchanged.

Three tests cover the floor, an image beyond it, and a larger configured budget.

🤖 Generated with Claude Code

[github-actions]

Perf comparison — this change, against its base

3 run(s) of each side, alternating on one machine. Allocation is a count and is gated exactly. Time is a measurement, so its gate is derived from the spread this job just observed in itself rather than fixed in advance.

corpus	bytes/char	gen0/Mchar	ns/char	Δ time	noise	gate
scroll-ascii	0.00 → 0.00	0.00 → 0.00	3.38 → 3.35	-0.8%	±1%	4%
sgr-churn	0.00 → 0.00	0.00 → 0.00	9.09 → 9.08	-0.1%	±1%	4%
truecolor	0.00 → 0.00	0.00 → 0.00	10.18 → 10.19	+0.1%	±3%	8%
alt-redraw	0.00 → 0.00	0.00 → 0.00	14.41 → 14.47	+0.4%	±4%	12%
unicode	7.66 → 7.66	0.45 → 0.45	35.35 → 35.78	+1.2%	±4%	13%
flood	0.00 → 0.00	0.00 → 0.00	94.52 → 95.18	+0.7%	±1%	4%

Each corpus is gated at max(4%, 3 × its own noise). A wide noise column means this runner was busy and the timing half of the table should be read as advisory; the allocation half is exact either way.

assemblies measured

• base: XTerm.NET 2.0.5.0 mvid:b94dd2c9-f50b-43db-b2d0-05a1b99d4b78
• head: XTerm.NET 2.0.5.0 mvid:a83e726a-a65b-4bb4-b367-f6e957ea504e

---

Perf comparison — cumulative, everything since 2.0.3

3 run(s) of each side, alternating on one machine. Allocation is a count and is gated exactly. Time is a measurement, so its gate is derived from the spread this job just observed in itself rather than fixed in advance.

corpus	bytes/char	gen0/Mchar	ns/char	Δ time	noise	gate
scroll-ascii	0.00 → 0.00	0.00 → 0.00	3.38 → 3.35	-0.9%	±1%	5%
sgr-churn	0.00 → 0.00	0.00 → 0.00	9.08 → 9.08	+0.0%	±2%	6%
truecolor	0.00 → 0.00	0.00 → 0.00	9.93 → 10.19	+2.7%	±3%	9%
alt-redraw	0.00 → 0.00	0.00 → 0.00	14.36 → 14.47	+0.8%	±4%	12%
unicode	7.66 → 7.66	0.45 → 0.45	35.80 → 35.78	-0.0%	±4%	13%
flood	0.00 → 0.00	0.00 → 0.00	94.48 → 95.18	+0.7%	±1%	5%

Each corpus is gated at max(5%, 3 × its own noise). A wide noise column means this runner was busy and the timing half of the table should be read as advisory; the allocation half is exact either way.

assemblies measured

• base: XTerm.NET 2.0.3.0 mvid:9ba0ce98-0d1e-4424-8aa1-9ab012cdc07a
• head: XTerm.NET 2.0.5.0 mvid:a83e726a-a65b-4bb4-b367-f6e957ea504e