Make the sequencer pixel generator cheaper on opened borders

[highbyte]

Summary

The sequencer pixel generator runs its per-cycle pipeline over every cycle of every line, so a frame that opens the vertical border, where the whole line is output, is about twice the work of a text screen. On a sprites-only demo that does so the sequencer rendered at three times the legacy generator's cost. Four changes to the generator, none of which change its output:

• Identical-block cache: a cycle whose fetched bytes, mode and pipeline state (shift register, its matrix and colour, pixel value, pair phase, XSCROLL load pixel) equal the previous block's copies its eight foreground/background codes and takes that block's end state. Covers the idle byte across an opened border, runs of identical cells, and blank cells under any XSCROLL, which the aligned fast path never reached.
• The line's colour resolve copies such blocks' colours instead of looking them up, via a per-line serial mark.
• Sprite rows are decoded into a buffer and written as runs through the bulk pixel delegates instead of a delegate call per pixel.
• The fetch ring's slots are derived instead of taken as three modulos per cycle.

Validation

• Timing on this machine (MacBook Air M1, .NET 10.0.401, quiet, headless probe running "For Your Sprites Only" frames 400-1000, medians of alternating repeats): sequencer 1587 → 1172 µs per frame with per-line sprites on, 1404 → 1025 off; legacy generator 826 / 725; CPU and VIC-II alone 404.
• Frame benchmark RenderOnly rows, this branch vs the integration branch alternating: 474-477 vs 476-489 without sprites, 491-494 vs 495-496 with eight, i.e. level. An earlier version of the cache had cost the text screen 7-10%; packing the compare into one key and using direct copies removed that.
• Output identical: 113 VICE test programs across ten suites with the same 18 known differences as before, 467 C64 tests, full suite 2935 passed, Commando and Giana Sisters byte-identical to the integration branch, desktop and browser apps build clean.
• SonarCloud branch analysis clean at Major and above.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud