fix(gs): correct depth write for all AFAIL alpha-fail modes

[smmathews]

fix(gs): correct depth write for all AFAIL alpha-fail modes

Corrected premise

This PR fixes a defect in the alpha-test-fail (AFAIL) truth table that sits next to the depth logic.

The bug

TEST.AFAIL documents four outcomes for a fragment that fails the alpha
test:

AFAIL	color write	depth write
KEEP (0)	no	no
FB_ONLY (1)	yes	no
ZB_ONLY (2)	no	yes
RGB_ONLY (3)	rgb only	no

classifyAlphaTest in ps2xRuntime/src/lib/ps2_gs_rasterizer.cpp used a
single writeFramebuffer flag to stand in for both the color and depth
disposition, and writePixel bailed out early only when that flag was
false. Every AFAIL case that survived the early-out then fell through to
an unconditional depth write (gated only by ZMSK). That made every
non-KEEP case except ZB_ONLY write depth on alpha-fail, against the table:
FB_ONLY and RGB_ONLY wrote Z when the table says they should not, and
ZB_ONLY (folded into the same branch as KEEP) wrote neither color nor its
required Z.

The fix

AlphaTestResult gains an independent writeDepth field so each AFAIL
case states its own color and depth disposition, instead of a single
writeFramebuffer flag standing in for both. writePixel gates the color
WriteVram call on writeFramebuffer and the depth WriteVram call on
writeDepth separately. The Z test itself is unchanged: a fragment that
fails the depth test still writes nothing, matching a passing fragment's
depth-test behavior.

A normal alpha-test pass still writes both color and depth. KEEP is
unchanged (it hits the early-out with both flags false, same as before).
FB_ONLY and RGB_ONLY now stop writing depth on alpha-fail, and ZB_ONLY
gains its previously-missing depth-only write.

Test

Three regression tests sit next to the existing AFAIL FB_ONLY and RGB_ONLY
framebuffer pins, each programming ZMSK=0 with a Z page distinct from the
color page and a pre-seeded depth sentinel so the depth path is actually
exercised:

• AFAIL=ZB_ONLY: forces the alpha test to fail with ZTST=ALWAYS, asserts
  the framebuffer stays untouched while depth is overwritten with the
  drawn fragment's z.
• AFAIL=FB_ONLY: same setup, asserts the framebuffer is overwritten with
  the drawn color while depth stays at its pre-seeded sentinel.
• AFAIL=RGB_ONLY: same setup, asserts RGB is written and destination alpha
  preserved while depth stays at its pre-seeded sentinel.

Repro

From the repository root:

cmake -S . -B build -DCMAKE_CXX_FLAGS="-msse4.1 -include cstdint" -DCMAKE_C_FLAGS=-msse4.1
cmake --build build --target ps2x_tests
./build/ps2xTest/ps2x_tests