Make the sliding 4-bit window the DHGR colour default

[st3fan]

Make the sliding 4-bit window the DHGR colour default

Field report: Thexder's DHGR title via Total Replay on the //e "looked like it stayed in HGR". The debugging split the question cleanly:

1. Soft-switch probe at title time: Graphics + HIRES + col80=true + dhires=true — Total Replay detected 128K and engaged DHGR correctly. The emulation core was right.
2. The renderer was the problem: our aligned-cell DHGR decode (the 6b starting choice, explicitly marked "revisit after a known DHGR image") turns hard edges into 4-px colour blocks — the title's credits render as unreadable pink mottling.

The fix — the parked experiment, promoted

DhgrColorMode { Aligned, Sliding } lands in scr.rs with Sliding as the default: each pixel's colour comes from the trailing 4-bit window, phase-weighted (1 << (pos & 3)), reducing exactly to the aligned value at cell boundaries. Side-by-side Thexder renders were night and day — aligned: mottled noise; sliding: crisp white text with thin composite-style edge fringes, exactly what a real monitor shows. Aligned stays selectable via set_dhgr_color_mode.

New end-to-end gate

two_e_totalreplay.rs: boots Total Replay on the //e (HDD + ProDOS + 128K detection), launches Thexder via the menu's incremental search, and asserts DHGR engages — the full stack in one test. Skips when the untracked .hdv is absent (the protected-WOZ pattern).

Verification

• fmt / clippy -D warnings / full cargo test green (32 result files).
• The mono DHGR path and its golden are untouched (mono never used the colour decode).
• colour_cell_selects_the_palette pins itself to Aligned (it tests the palette mapping); a new test asserts the sliding default keeps white runs white.
• Visual proof: the two Thexder renders in the conversation — the owner can re-check live with two --model 2e --color --hdd "disks/Total Replay v6.0.1.hdv".

🤖 Generated with Claude Code

[st3fan]

Build metrics — DHGR sliding-window promotion

Model	Claude Fable 5
Thinking mode	adaptive

Token usage for this PR (delta since PR #244; covers the /RAM and ProDrive explanations, the Thexder debugging session — headless reproduction, ][+ vs //e comparison, side-by-side renders — and the promotion):

Metric	Tokens
Input	242
Output	106,135
Cache write	1,969,544
Cache read	71,427,300
Total	~73.5M

Cost: ~$101.36 _{(Fable 5 @ $10/$50 per M in/out, $12.50/$1.00 per M cache write/read)}